Well at least he wasn't shacked up with one o' those n!g93rs
hahaha
I don't agree with the statement, just wanted to say it...and in my head so that you know, I was sayin' it in JK Simmons voice.

From the perspective of myself, if I were the straight guy parent that I'll most likely stay, I'd personally wanna talk to the "son" first, ask if he'd shared my observation about the person in question, ask is he's cool with that.
If he wasn't, even for the reason "I don't feel comfortable around gay guys" I'd then be cool with a room change - fact is, it's not just the occasional walk-by or hang-out, it's living together, huge difference.

Hell, I'm not ashamed to admit, while I'm pretty much neutral about gay guys, (though I may not understand why a man OR woman finds males attractive) I don't feel entirely 100% comfortable around 'em; no negativity there, just generally speaking that's how it turns out, I've known gay guys, but have never really gone past general acquaintance. Gay chicks, well the ones that don't outrightly hate males I usually get on pretty well with, see their attraction I can share and understand (bulldykes aside), that and they tend to be a bit more towards guys to talk to.

If he was cool with it, I'd talk to the gay dude with him...ask if he was cool with it also, and if so, whatever the fuck - I imagine respect based ground rules on the basis of each respective flatmates sexual encounters/"me" time, would be established early, and if it doesn't work out for one reason or another, I'd encourage both/either of 'em to go ahead and do what they're right with.
If it's a college/uni, neither would need any extra pressure that would come from being around someone they're not comfortable around.

But yeah, on the parental side - not really the parents choice.
But for me, a parental responsibility will be to ensure my child is able to live within their comfort zone, and that they make the decision where/what that is - I will act accordingly.

no I'm not;)

I agree with this in that if they do it to avoid grouping possible sexual distractions (be that right or wrong) then it is similar to having a boy/girl share a room. Whos is to say the so is straight (or more likely up for a bit of collage days experimentation :-)

Plain old ignorance. Mommy's afraid of her son turning.