They way they handled Steve was respectful and extremely well done when it came to him learning to deal with the loss of his husband and I'm always for second loves and lost lenores in fiction.
But it just felt... I dunno, that the time frame for ME3 was not long enough that I could see that relationship develop in that direction as it did? I know that people hook-up more quickly in wartime, but it just seemed that the relationship moved too fast from consoling a grieving widower to the bedroom.
In ME2, I appreciated that Jack was the only one who didn't have a sex scene, as it was clear that her emotional baggage meant that she wasn't prepared to be intimate with Shepard just yet, although she was by the time of ME3.
Steve would have been even better if he had been introduced in ME2, come to think about it, since we never do see our invisible shuttle pilot. We'd witness the loss of his husband in Horizon, as the guy piloting the shuttle during the Collector abduction he'd be the only other Normandy crewman besides Joker left who canonically survived, and his friendship/relationship with Shepard would have had time over two games with which to span.
Otherwise, having his husband die on Eden Prime would have worked as it'd given him more time to come to terms with the loss (but not overcome the obvious grief) before he might be ready to enter into a relationship with Shepard. It'd have also given him more interaction with Joker, Kaidan and Ashley, who were all present during the Eden Prime mission, as well as Jacob (if Steve had first appeared in ME2).
It bothers me as well that some people just look at Steve as being "the gay LI with a dead husband", when there is more to him as you say and he's a great character in his own right.
I always enjoyed going down to the cargo bay to chat and he felt like a natural addition to the crew, someone who could have easily been there since day one... if only Engineer Adams had been that chatty, now I think about it?
My sole complaint about the romance arc is that I personally cannot shake the feeling that Shepard might have been taking advantage of his grief and that Steve might not be ready in the right place to pursue this relationship, even if he thinks he is.