Really, if the man is not a widower then there is no good reason for an unrelated woman to know his possible sexual history without a statement of intention to check suitability for marriage with one another.
It's not modest to go around talking about private or occult sins without a good reason...and just meeting someone online who might POSSIBLY be interested in getting to know you is not a good reason.
If a man or a woman is interested in checking his/her suitability for marriage with the other, then the intention ought to be stated, and safeguards put up to discuss things of a private nature without sin...otherwise it's just vain prattling and someone is going to get emotionally hurt.
Try asking him 'fair game questions', that are often asked by everyone, very early in dating:
'Have you ever been in a serious relationship?' (or long term relationship)
'Have you ever lived with someone?'
(((If there has been a lot of physical contact/opportunity, most don't have the ability to avoid the natural progression of physical contact to sexual contact. So most people would assume a 'long term relationship', means there was a sexual relationship. Or a deep, emotional attachment, also involved sensual/sexual involvement.))) Of course, it's a whole nother' thing to try to uncover 'one night stands', basic promiscuity etc. PS STD's also are found/transferred thru the mouth/throat, so kissing has it's risks.
((In the case of a virgin, you'd want to find out simply, if the other person is one too. It's a fair issue, considering there's a very high value on it. It's a yes or no answer. But you don't have to ask a person outright. Look at the persons actions. Actions will be the best clue, very early on, of the true virtues they possess. Chastity is practiced, it's a behavior. Chastity is expected of everyone (single, married or religious) (((i.e. married people are expected to remain chaste too, to be loyal to their spouse and to not have sex with others))) If your date, doesn't show indications of protecting your chastity, it's likely they aren't chaste, themselves. ((There are hundreds of examples......from things as simple as respectful touching only, curtailing physical excitement, discouraging excessive intoxication, limiting intimate opportunities, planning dates and activities that don't have sexual connotations...)) (((The sexually active will do just the opposite, they will seek opportunities for sexual contact)))
Even if a person doesn't have a sexual history, it doesn't mean they don't have other issues that would weaken a healthy sexual relationship (i.e. lust, porn, mastxxx w/out sex...)
Finding the right spouse, for a long term, exclusive, highly mutually satisfying intimate relationship requires patience (because there are so many people looking to marry and offering themselves as marriage potentials). And also, the ability to discern who the other person really is, which takes place during 'dating'. Who are they?? Are they right for you? And who you? And are you right for them? You're each a bit of a matchmaker for the other.
A great marriage, means it's a great match, for both of you, not just one of you.