If you mean, "have intercourse", that can, of course, be done with a stranger...and nothing is further from marriage.
You can also be married a long (and happy) time without ever really managing to infiltrate who the other person is.
If you are lucky you really can become one heart, one mind. But I don't think many achieve it.
EXPLANATION OF THE TEN COMMANDMENTS
Articles 3-12 translated by translated by Joseph B. Collins, New
Edited, with prologue and articles 1-2 added, and html formated
by Joseph Kenny, O.P.
THE SIXTH COMMANDMENT
You Shall Not Commit Adultery.
After the prohibition of murder, adultery is forbidden. This is fitting, since husband and wife are as one body. They shall be, says the Lord, two in one flesh [Gen 2:24]. Therefore, after an injury inflicted upon a man in his own person, none is so grave as that which is inflicted upon a person with whom one is joined.
Adultery is forbidden both to the wife and the husband. We shall first consider the adultery of the wife, since in this seems to lie the greater sin, for a wife who commits adultery is guilty of three grave sins, which are implied in the following words: So every woman who leaves her husband,... first, is unfaithful to the law of the Most High; and secondly, she has offended against her husband; thirdly, she has fornicated in adultery, and hath gotten her children of another man [Sir 23:32-33].
First, therefore, she has sinned by lack of faith, since she is unfaithful to the law wherein God has forbidden adultery. Moreover, she has spurned the ordinance of God: What therefore God has joined together, let no man put asunder [Mt 19:6]. And also she has sinned against the institution or Sacrament. Because marriage is contracted before the eyes of the Church, and thereupon God is called, as it were, to witness a bond of fidelity which must be kept: The Lord hath been witness between you and the wife of your youth whom you have despised [Mal 2:14]. Therefore, she has sinned against the law of God, against a precept of the Church and against a Sacrament of God.
Secondly, she sins by infidelity because she has betrayed her husband: The wife does not have power over her own body: but the husband [1 Cor 7:4]. In fact, without the consent of the husband she cannot observe chastity. If adultery is committed, then, an act of treachery is perpetrated in that the wife gives herself to another, just as if a servant gave himself to another master: She forsakes the guide of her youth, and has forgotten the covenant of her God [Prov 2:17-18].
Thirdly, the adulteress commits the sin of theft in that she brings forth children from a man not her husband; and this is a most grave theft in that she expends her heredity upon children not her husbands. Let it be noted that such a one should encourage her children to enter religion, or upon such a walk of life that they do not succeed in the property of her husband. Therefore, an adulteress is guilty of sacrilege, treachery and theft.
Husbands, however, do not sin any less than wives, although they sometimes may salve themselves to the contrary. This is clear for three reasons. First, because of the equality which holds between husband and wife, for the husband also does not have power over his own body, but the wife [1 Cor 7:4]. Therefore, as far as the rights of matrimony are concerned, one cannot act without the consent of the other. As an indication of this, God did not form woman from the foot or from the head, but from the rib of the man. Now, marriage was at no time a perfect state until the law of Christ came, because the Jew could have many wives, but a wife could not have many husbands; hence, equality did not exist.
The second reason is because strength is a special quality of the man, while the passion proper to the woman is concupiscence: You husbands, likewise dwelling with them according to knowledge, giving honor to the female as to the weaker vessel [1 Pt 3:7]. Therefore, if you ask from your wife what you do not keep yourself, then you are unfaithful. The third reason is from the authority of the husband. For the husband is head of the wife, and as it is said: Women may not speak in the church,... if they would learn anything, let them ask their husbands at home . The husband is the teacher of his wife, and God, therefore, gave the Commandment to the husband. Now, as regards fulfillment of their duties, a priest who fails is more guilty than a layman, and a bishop more than a priest, because it is especially incumbent upon them to teach others. In like manner, the husband that commits adultery breaks faith by not obeying that which he ought. Why adultery and fornication must be avoided
Thus, God forbids adultery both to men and women. Now, it must be known that, although some believe that adultery is a sin, yet they do not believe that simple fornication is a mortal sin. Against them stand the words of St. Paul: For fornicators and adulterers God will judge [Hb 13:4]. And: Do not err: neither fornicators... nor adulterers, nor the effeminate, nor liers with men shall possess the kingdom of God [1 Cor 6:9]. But one is not excluded from the kingdom of God except by mortal sin; therefore, fornication is a mortal sin.
But one might say that there is no reason why fornication should be a mortal sin, since the body of the wife is not given, as in adultery. I say, however, if the body of the wife is not given, nevertheless, there is given the body of Christ which was given to the husband when he was sanctified in Baptism. If, then, one must not betray his wife, with much more reason must he not be unfaithful to Christ: Know you not that your bodies are the members of Christ? Shall I then take the members of Christ and make them the members of a harlot? God forbid!. It is heretical to say that fornication is not a mortal sin.
Moreover, it must be known that the Commandment, You shall not commit adultery, not only forbids adultery but also every form of immodesty and impurity. There are some who say that intercourse between married persons is not devoid of sin. But this is heretical, for the Apostle says: Let marriage be honorable in all and the bed undefiled [Hb 13:4]. Not only is it devoid of sin, but for those in the state of grace it is meritorious for eternal life. Sometimes, however, it may be a venial sin, sometimes a mortal sin. When it is had with the intention of bringing forth offspring, it is an act of virtue. When it is had with the intent of rendering mutual comfort, it is an act of justice. When it is a cause of exciting lust, although within the limits of marriage, it is a venial sin; and when it goes beyond these limits, so as to intend intercourse with another if possible, it would be a mortal sin.
Adultery and fornication are forbidden for a number of reasons. First of all, because they destroy the soul: He who is an adulterer has no sense, for the folly of his heart shall destroy his own soul [Prov 6:32]. It says: for the folly of his heart, which is whenever the flesh dominates the spirit. Secondly, they deprive one of life; for one guilty of such should die according to the Law, as we read in Leviticus (20:10) and Deuteronomy (22:22). Sometimes the guilty one is not punished now bodily, which is to his disadvantage since punishment of the body may be borne with patience and is conducive to the remission of sins; but nevertheless he shall be punished in the future life. Thirdly, these sins consume his substance, just as happened to the prodigal son in that he wasted his substance living riotously [Lk 15:13]. Do not give your soul to harlots, lest you destroy your inheritance [Sir 9:6]. Fourthly, they defile the offspring: The children of adulterers shall not come to perfection, and the seed of the unlawful bed shall be rooted out. And if they live long they shall be nothing regarded, and their last old age shall be without honor [Wis 3:16-17]. And again: Otherwise your children should be unclean; but now they are holy [1 Cor 7:14]. Thus, they are never honored in the Church, but if they are clerics their dishonor may go without shame. Fifthly, these sins take away ones honor, and this especially is applicable to women: Every woman who is a harlot shall be trodden upon as dung in the way [Sir 9:10]. And of the husband it is said: He gathers to himself shame and dishonor, and his reproach shall not be blotted out [Prov 6:33].
St. Gregory says that sins of the flesh are more shameful and less blameful than those of the spirit, and the reason is because they are common to the beasts: Man when he was in honor did not understand; and became like senseless beasts that perish [Ps 48:21].
THAT is why the Church frowns on extra-marital sex:
a -- one person is not getting what she thinks she's getting
b -- one person is indulging in a physical act without any commitment to the consequences
In short, sex is being misused.
* Talking in generalities, of course.