Well, the name of the topic is "Would you date Shepard in real life? What makes Shepard attractive as a person?" so the waifu/husbando part kind of comes to mind then when analysing the character, I guess. If that was not the point, then I apologise for bringing it up.
Frankly, I'd say: Yeah, of course they're attracted to him because that's how the writers catered to the player, having the male player specifically in mind. Then it doesn't matter whether the character has the personality of a cardboard cutout or not. And, honestly, no, I don't think any character should be universally considered appealing. If that happens, especially if it happens a lot, it's cheap. Simple as that. Especially if it's not backed up by appropriate reasons. At least sometimes they should work for it. At least sometimes they should run into an unbreakable wall. Otherwise it pushes what one can believe. Not even popular models, singers, actors, etc. are universally loved by everyone.
However, we can't forget that Shepard is sort of a blank slate character that is under our control. I don't think Shepard was ever meant to be a fully fleshed out character in the first place, which makes him difficult to judge in this aspect. I mean, it's a character we're supposed to roleplay. He has some basic personality traits among which we can choose and some we cannot choose. The rest, we're supposed to decide what he's like. So some vacuousness has to be expected. It gives the player space. It's a double-edged sword if one happens not to like that kind of thing.
I think it's also a big difference whether we talk physical attractiveness, as you mention earlier, or personality's appeal, as you mention later. They don't have to be one and the same thing. And if it comes to personality, I think my earlier post still stands. He can be bland but still be a sweet person that wants to make the galaxy a better place. Or not. That's up to the player to decide. But yeah, one type of personality definitely should not appeal to everyone.
You know, though, it's sort of interesting that femShep can be refused a few times throughout the game - by Joker, by James, don't let me start about Jacob. Are there any cases like that in case of manShep? (I'm honestly asking, I don't know.) And I think that's a good thing. Although I do not like that it's only the love interests for femShep that ever get unavoidably lost.