It's very sad how everyone seems to think romance culminates in sex.
I think the amount of sexual repression we live with is sad. In my opinion, a romantic relationship without sex would be an inheirantly unhealthy one.
With Tali, it's not just that you can't have sex. It's that you can't touch her at all except through her environmental suit. No kissing, no carressing, no physical expression of intimacy at all that she can actually feel. Shepard is human. Like all humans, he has physical needs. You can ignore them for a while, but to deny them completely is unhealthy. There are plenty of other women that Shepard can touch. Whether he does so or not, Tali would envy them that. Bitterness would grow between them, and the whole relationship would eventually fall apart.
I like Tali, but unless you are also a Quarian, I don't think a romance with her is feasible.