Honestly ever relationship on this show has shady undertones:
Will constantly uses the kids to get into Emma's pants.
Santana has manipulated Brittany in the past and called her stupid without actually using the word.
Blaine was willing to throw all of his morals out the window in the backseat of a car for a play. Kurt often gets jealous of Blaine when Blaine gets something he doesn't.
Finn's always constantly forgetting something important about Rachel like that time he fed her meat forgetting she was a vegetarian.
The only healthy non shady relationship on this show is Tina/Mike.
Can't really argue with any of this. Will gets downright creepy at times how he's so determined to take Emma's virginity. Hell, the entire Rocky Horror Glee Show was a convoluted attempt by Will at stealing Emma away from Carl using the entire glee club as his pawns.
The way Santana takes advantage of Brittany's lack of intelligence to manipulate her gives Brittana a really shady undertone, IMO, probably the most of any relationship on the show.
Finn feeding Rachel meat was epic fail on his part. But Rachel's also gotten jealous and threatened any time he's been more popular than her.
Quinn uses her boyfriends for popularity points and pretty much ends up hurting any guy who gets involved with her.
I think both Rachel and Kurt have issues with being too self-centered to put themselves aside and be supportive of Finn and Blaine when they should be instead of being jealous of them, although Kurt did suppress his jealousy about Tony to support Blaine, which was a nice unexpectedly mature move on his part.
Blaine has...judgment issues. Although overall he's a better boyfriend than Finn, who is pretty much walking Fail 2/3 of the time.
For the most part, Mike, Blaine, and Sam are all decent boyfriends, and Mike/Tina, for how "background" they are, have the longest-running and apparently most stable relationship on the show.