Personally, I'd like to be able to romance Mako on my female BH. I never really felt comfortable with Torian. He just isn't her "type".
If you look at the different companions out there, a lot of the male companions are odd alien types, which makes them less likely to be a romance option, while most of the females are near-human and at least somewhat attractive. So when it comes to the idea of male/male romance, obviously you'd have to leave out Quezen, Gus, Skadge , Blizz, Khem, Xalek, Bowdaar, Broonmark, Yuun, any droids, etc..
Some of the other humanoid types that have no current romance options could be a maybe, like Zenith, Gault, Tanno, Scourge? (lol)
So there's Doc, Torian, Quinn, Vector, Lt. Pierce, Corso, Doc, Andronikos, Tharan, Iresso, Talos,
I'm not really familiar with all of them, but I feel like Doc would be out, and Tharan clearly only loves holograms. I almost felt like Torian might have at one point been considered as a SGR.
I'm speaking theoretically here, I'd be surprised if they went back and made old companions SGR compatible any time soon.
Basically the same with my BH. The only line I felt worked for my BH and Torian, is when she called him arm candy.
But where as Torian is arm candy and a friend to her (in my head anyways), Mako is the arm candy and so much more.
I can see them adding SGR in with companions, I just don't see them doing it in one big patch, and it just seems easier to do it piecemeal. But I admit this is me thinking the dialogue isn't in game for SGR and they'd have to get the voice actors back to rework the scenes or add in new ones. I also don't know how expensive that option is or if it's even a possibility to do (which may be why current companions don't become an option).