If my daughter, when she is a senior in high school and has been dating a solid, upstanding boy for a year, I understand that she may have sex with him.
And no, I do not set different standards for male and female kids.
If you set the speed limit at 55, many people will drive 70. If you set the speed limit at 65, many people will drive 80, and more will be driving at 80 than if the speed limit is 55. So if you start with the expectation that teenagers will have sex, more will have sex than if you start with the expectation thta they won't. And given that despite years of condom education and everything short of teachers following kids home and putting them on for them, we still have high STD rates, cervical cancer problems, and plenty of abortions, I don't think it's any more reasonable to expect them to use a condom or other birth control every time than it is for them to not have sex. And another deterrence is the expectation that kids understand and deal with the consequences.
Even if your son uses a condom, it could fail. If it does and his girlfriend gets pregnant, is he going to marry her? Pay child support? Agree with her to give the kid up for adoption? Or will she abort your grandchild, even if you son doesn't want her to? And even if nothing happens, what if they break up? Break-ups are rarely unilateral and often hurt one party more than the other. Sure people survive that but part of the point of Mirriam Gross' book is that a lot of young women get very messed up by having sex and then being dumped by the guy, because girls tend to bond with the men they have sex with.
And while you certainly know your son better than I do so I'll take your word for it if you think I'm wrong, but I can't help but thinking that if a teenager comes to their parent telling them that they might not be able to remain abstinent that they are looking for encouragement or help staying on course and not a sudden shift in expectations to a green light with qualifications. Is it possible your son was looking for "Hang in there!" instead of "Use a condom."?