OP you really are not making any sense at all.
Some people regard sex as being an important relationship milestone and a marker of the relationship being serious which is why they choose to wait (in some cases until marriage). These people will usually meet each other's families long before they have sex.
Other people think having had sex or not is no indication of whether a relationship is serious or long term. These people will usually have sex long before they start thinking about meeting each other's families.
You seem to be muddling the two ideas here.
You seem to think that sex is an important relationship milestone and that your daughter's relationship can't possibly be serious until they've had sex, and simultaneously think that meeting your partner's family is an important relationship milestone and almost tantamount to an engagement, making it much more difficult if you then later break up. If you think both having sex and meeting the family are important milestones indicating that the relationship is serious, fine, but one of those two things has to come first.
I've never waited five months in a relationship before having sex personally, but I would have thought that statistically, relationships where the couple didn't have sex right at the beginning are probably more likely to last the distance, simply because they waited until the relationship was already serious before having sex, rather than having sex and waiting to see if the relationship became serious.
Finally, if I could go back and tell my 17 year old self anything, it would be that how well you get on with each other's families is a far greater measure of compatibility than your sex life. Perhaps your daughter's boyfriend wants to check that her family is one he wouldn't mind joining before he gets seriously involved with her. Don't screw it up for them by being weird.