Just be wary of implying that he shouldn't have different, straight individual boys sleeping over either, just because he's gay and they're a boy
^Doesn’t make sense.
Straight firm and straight boy. 14. But not in a relationship, just friends. Would you allow them to sleep in same room together?^
Straight boy and straight girl - you can assume they might both in theory be interested, so them not being openly in a relationship is less of a protection.
Both gay and of the same sex - again, you can assume they might both be interested.
Straight boy and lesbian, or gay boy and straight boy, or any other combination where one is in theory not the other's type - you can ban sleepovers of course for better safe than sorry, but it's harder to explain without implying either that one or other of them might be not as gay/straight as they are saying, OR that eg the gay boy might try something on with the straight boy even though that boy is straight. That latter situation is exactly what gay people sometimes face straight people worrying about, so it's probably best not to imply you think that might happen. I would still do the door open thing, at the very least, but I would be careful how I presented it.
Mixed sex would be trickier though... even if the risk of secret shagging between a mixed sex couple with seemingly nonmatching orientations is theoretically low, the consequences are potentially much worse.
Basically I wouldn't rely on any pair of teenagers orientations not matching as protection against secret shagging, or their relationship status, but I would be very careful about accidentally implying my gay teen might make a move on a straight friend if sleepovers in their room are allowed.