I think it’s a combination of a few things, and the same attitude that applies to Forsaken.
One, they’re undead, which brings to mind some pretty nasty mental imagery as far as physical intimacy is concerned. What’s their state of decay? How bad do they smell? Are they infested with anything? What sort of diseases do they carry? A huge focus on the class, even those not of the unholy school, is “disease”. So as far as relationships with the living go, people go straight to imagining worst-case scenarios.
Two, they have issues. They’re plagued by a constant drive to kill everything around them, which makes even being friends with one a risky business, and they’ve been through some exceedingly heavy emotional trauma that (in most cases) makes them pretty distant and not the most cheerful or affectionate people. People tend to view them as emotionless, which I’ve argued against here. Some may be, but it’s not a definitive trait. Because of these things, people tend to think they’d not desire companionship.
They’re a diverse group. Some were raised immediately after their death, others may have been left out in the sun for a few weeks, and others may have never even “died” and just got corrupted into undeath by necromancy use (a la Arthas). Some embrace what they are, others struggle to connect more with the living. Outside of a few blanket class traits like the hunger or undeath, you can’t make very many “DKs can’t—” statements because they’re all different.
Intimacy with one is a risk no matter how clean and well-adjusted they are, and I quite frankly can’t see the fun of RPing a relationship with a DK that’s all rainbows and sunshine (why are you playing a DK?), but I don’t think it’s impossible and definitely not worth any amount of knicker-twisting.
If you adamantly question why someone you don’t even know wants their DK to get a boner because ~undead can’t do that~ but are perfectly accepting of Forsaken being able to move their elbows despite no muscle or flesh being attached to them, this is probably you:
