Okay but what if I were to say that Theo lost his virginity to Boris...
This of course depends on how you define sex, but I feel it’s old fashioned and close minded to say it’s only penetration.
Sex is any physical sexual activity performed by two or more people. As far as we know, (Theo could’ve always left out some stuff) they did definitely jack each other off, and I absolutely consider that to be sex. Theo doesn’t mention having had sex with anyone before that either, so yes it would seem that he lost his virginity to Boris.
Now if you meant the conventional “method” of losing it, I don’t think that that ever happened. They were never ready to go “that far”, because it would’ve been too gay. After all, anal penetration is conventionally considered (gay) sex, but handjobs aren’t. They could always brush off their intimacy, as long as it was just “boys being boys”, rather than actual “sex”.
Is there a chance Theo repressed it or just didn’t tell us? Yes, always. But do I think it’s likely? No.