Putting it that way, one might take it to say that if they have made a mistake, God did not protect them from that situation. The spirit is willing, but the flesh is weak. If God did not allow us to be tempted, then we would never have the opportunity to rise above temptation and grow. There are situations where someone may not have had a choice in the matter. But the more common issues of premarital sex involve the human will. We are given the opportunity to choose to say no, or to say yes. This goes for everything, not just premarital sex.
What your'e talking about is God's permissive will. Where he doesn't actually will such temptation but allows it.
You run into some nasty theological answers if one thinks that God actually wills our suffering and temptation; such behaviour is not loving and is rather nasty and vindicitive.
Sexual sins in particular stem from our inherent nature being wounded by sin. God built our bodies, giving us all those neat little hormones that encourage us towards procreation. But sex out of marriage is harmful and damaging to our souls and bodies. We are sexually tempted by others when they utlise their free will to dress immodestly or to pursue unnatural and deviant sexual choices. We can either give in to the flesh or we can refuse it, understanding sex belongs between one man and one woman in marriage, procreative and unitive.
We suffer because of the sins and choices of others, and ourselves. God doesn't place temptation before us, satan and the sins of others do. God, however, allows this because he will not interfer with free will.
Fr. Robert Barron has a good explanation, contrasting the issue of chaos and sin and God's allowance of such as a painter painting an image. IT's not perfect as soon as the first brush stroke is made, it takes time, and some of those mometns will look chaotic.
God allows the sins of others and temptations, not for their own sake to trap us, but to enable us to step up and move beyond it.
Permissive will, not direct will.