I mean, before Magnus realized Alec wasn't out, he hit on him pretty hard and obvious for everyone around. But (speaking from personal experience) being closeted, doesn't necessarily equal having no experience at all, so I'm not surprised Magnus didn't assume, Alec never had a relationship. I think a lot of us dated or were intimate with ppl in secret before coming out, it's not uncommon at all. Like Magnus said, Shadowhunters are a don't ask/don't tell culture, and even offered to date secretly
Yeah, many of us date on the down-low, etc., so I don’t see it as an odd assumption for Magnus to have made. Though the more I look at the scene, the more I do think the writers went a little out of their way to make it really clear. And I don’t know if it’s because of the amount of people up to even last week who kept looking for reasons as to why Alec should’ve had more experience in the past, and by reasons I mean they all basically amounted to “Alec doesn’t look/act like a virgin” which was frankly terrible. Like, what does one look or act like first of all, and that’s not even getting into the fact that it’s a social construct, anyway. We all have different levels of intimacy that ultimately counts as a sexual experience for us; this isn’t something other people should decide for you.
But regardless, the underlining of Alec as a man with little relationship experience combined with his being treated without the shy blushing trope or whatever was really, really needed, especially because it’s exactly this trope that caused people to accuse Magnus of being “predatory” in turn, as if Alec isn’t a perfectly grown ass adult who can take some sexual innuendo or blatant flirting. Or to write Magnus as a passive submissive boyfriend because, you know, Alec is delicate apparently or whatever. The trope is harmful all around and it really needed to be stopped and after what this episode established I hope I never encounter it again.