I... honestly don't mind it either way. When a character's sexuality is firmly rooted in the character, it does seem more realistic to me. But at the same time, games are there to explore fantasy. It would certainly suck, albeit in a very realistic manner, for me to really love one romance option, only to realize far too late to start over, that I picked the wrong gender for them.
I honestly can't say much for ME3's same sex romance options, as I played a female Shepard, and... given that I've been playing it since ME1, had already established a romance with Liara, which I didn't feel my character would compromise just because she happened across a human lesbian. Once I heard Kaiden was made bi, I considered making a male character to check that out, but... figured it wasn't worth the effort in the end.
Dragon Age I am much more familiar with. My first play through was as a male romancing Morrigan (though he had a little fling with Zevran, which he called off in favor of Morrigan), but on my second time around I had a female character who romanced Leliana.
On my first playthrough of Dragon Age II, my female Hawke romanced Isabela (which I might repeat my next game, but I'm also considering Merrill).
I can't say much for the gay MALE options in either case (I love Zevran to death, and if I play through yet again, I will most likely have a male character romance him properly, but I haven't gotten around to it; and... I just honestly cannot bring myself to create a male Hawke... his voice is just so... bland and uuuuugh. I couldn't make it through the whole game dealing with that dull monotone.)
And... I honestly don't mind them all being magically bisexual. I don't agree with everyone who says its a cop out or pandering or whatever. Given the amount of backstory and personality Bioware puts into its characters, I would never accuse them of making bland characters. The effort that goes into each one means it was likely MUCH easier to make them all love interests to either gender, rather than trying to make seperate gay and straight ones. Is it a bit lazy? Perhaps. But I don't think it detracts from the game experience significantly, and I still love the characters just as much.
I mean, ideally, I would love to see an RPG where there was a plethora of love interests, gay and straight to pick from, but I can understand why Bioware may opt to do it DA2 style. It delivers more choices with less work. Instead of having the ONE gay option and ONE lesbian option, who... hopefully you like. Otherwise you're screwed. You can chose from any of the love interests, whichever suits your style. I can enjoy that. I mean, as much as I love Isabela, I'm glad Merrill is a choice, because I like the idea of my female Hawke romancing her as well.