Steve Cortez and Samantha Traynor in ME3.
Kelly Chambers was in ME2.
That's a pretty gross simplification.
Take Mako as an example. The romance arc with her is part of her larger companion/character arc, which more or less spans the entire class story for the Bounty Hunter. That's just not at all comparable to a few flirts (and sex) with a character you'll never see after you've left that one planet.
Well, you do sorta see him later on in the storyline. And I really can't speak of Mako's romance storyline, as my BH is female. But I didn't think Andy's was much more than that.
I'd check with Torian, but I've decided my BH just wouldn't go that far with Torian. Flirt sure, but no relationship, I'll go with her having a secret crush on Mako that will likely never be filled.
And I have no intention of rolling a class twice just to see it.