Seems reasonable to me, until looked at from the perspective of.
you can be a lesbian, why not a gay guy?
Which is the entire argument to me.
I'd counter that with, 'is there any reason why he should possibly be gay?'
Sure I personally was never likely to make a gay Shepard... Because my Shepard is and always will be a bisexual woman.
That's beyond the point though.
Granted for the full experience of the game the relationships are key, but they're entirely optional.
You can go through the whole game without ever really trying to interact with anyone on the Normandy.
Besides the intriguing blue alien chick who my Shepard found strangely aluring and Wrex, a battle-hardened, like-minded companion my Shepard had exactly that kind of relationship with the majority of her crew.
I can understand the first reason because as much as some may have seen Mass Effect as a "Choose your own adventure" type of game it was a very fixed narrative.
Granted so was Dragon Age, but that gave us whole other levels of choice, which weren't present in Mass Effect.
So yeah, if they flat out believed when they wrote it that Shepard could never be a gay guy, then that's their choice.
They've proven with other games that they're not afraid of the idea. So I'm not seeing an issue.
If Homosexual relationships weren't possible in any Bioware game there'd be a complete argument.
So really, yeah it should be an option, but it's not really that important.
(At least in my eyes.)