Then explain what are they? In turm of personality.
Alistair is naive, immature, and uses his goofy humor as a self-defense mechanism. He's generally good natured, but is easily petulant if you disagree with him. Plus, you can 'harden' his personality to be more logical and decisive and less emotional. Once in the romance, he's a romantic, but it's less confident and more awkward and adolescent.
Kaidan is more of a "classic" leading man for women. He's genuinely nice, but he's also got an opinion. His sense of humor is more wry. He's smart and often offers a counterpoint to Shepard. He's supportive of Shepard, but will also challenge him/her if he disagrees with him/her. He's also got a romantic side, but is more confident and mature.
Carth is bitter, defensive, and aggressive at first. He has major trust issues and it takes quite some work from Revan to de-ice him. He's definitely more emotional than logical. He's got highs and lows throughout the relationship, going from being romantic and supportive, to aggressive and confrontational. He also suffers from Carth Syndrome, so a bulk of his romance is around his getting over his dead wife.
Sky is a bit like a combination of Kaidan and Carth. He's supportive and "nice" with a sense of humor, but he suffers HARD from Carth Syndrome (having not just a dead wife, but also a dead daughter). His romance, however, is less about "learning to trust again" and more about "learning to let go". Even though he's the least "original" of these guys, he actually came before Kaidan, so it's more accurate to say that Kaidan is a bit like Sky minus Carth.
Jacob is nothing like these other guys. He's not really emotional at all. I tried doing this romance once with one of my femShep's and it was pretty darn boring. It starts being pretty one-sided (with Shep being the aggressor), and I never really got the impression that Jacob was "in love" so much as "hooking up with" Shep. He's super professional and not emotional at all. In fact, it was like trying to get blood from a stone. To me, he's more of a supporting character than a leading man because he's just not that interesting and doesn't give much beyond the one questline with his father.
I think that these guys are all different. Their personalities are different. Their storylines are different. Their romance arcs are different.