Let me put it this way...
Good:
-Liara ME1
-Kaidan ME1
-Ashley ME1
-Liara ME2 (LOTSB)
-Thane ME2
-Garrus ME2
-Jack ME2 (Renegade in her quarters)
-Tali ME2
-Kaidan ME3
-Liara ME3
-Miranda ME3
-Jack ME3
-Thane ME3 (I guess lol)
-Tali ME3 (I'd imagine she has to keep the suit largely on, maybe)
-Garrus ME3
Bad:
-Miranda ME2
-Jacob ME2
-Jack ME2 (Para/Rene final bit)
-Ashley ME3
-Steve ME3 (yeah just pan away *entirely*, and not in a fluid way like other romances)
-Samantha ME3 (OH GOD THIS)
(Not getting into Citadel content because I don't feel like it)
(And I actually understand male characters wearing briefs, especially given graphical limitations lol; fully clothed has to make sense for the scene though)
(And I'm assuming MaleShep when possible here. If ME3 FemShep is involved, yeah, I find the bra super super silly. "Let me just put this back on for the camera...")
To me, it's not necessarily all about nudity or no nudity, explicit or not explicit. It's about appropriate and stylistically smart representation of actions and behaviors. When it gets to a comical, nonsensical, or seemingly stupid level, I can't like it.
I think Mass Effect romances, and heck, Bioware romances are mostly fine. However, each game has its issues. Sometimes a scene really seems to call for more nudity, else it goes wrong. Sometimes the use of any nudity feels pathetic to the scene.
It's like Bioware wants to pander to arousal in a silly way, without then showing natural nudity when that scene would make more sense with it. For example, I very much like Miranda's sex scene in ME3 compared to ME2. But no sex happened on screen? Yes. And that's quite fine. And then Miranda became one of my favorite romances in Citadel DLC, for using smarter script, clothing states, use of the camera, etc.
It's the whole mix of taste, realism, logic, emotion, eroticism, drama, and a bunch of other things.