While Tali does reek of pandering fan serviceness, I don't see why "a character who happens to be a virgin" is something to be gotten rid of. It's not like we haven't seen sexually adventurous female lis either.
I think the only fan-service-y thing about Tali's romance is the speed with which she, erm, adapts to Shepard. Other than that I find the exploration of an inter-species romance to be interesting, but more than that I think Tali's romance gives us insight into quarian culture. The quarians' developed aversion to foreign substances makes it so that the most intimate thing someone can do is link suit environments which - I get the feeling from Tali's dialogues - doesn't happen with many people for an individual quarian. If quarians don't link suit environments with many people, you can maybe extrapolate the cultural sexuality as well. Self-restraint has become cultural but was possibly born from actual physical necessity. The fact that Tali hasn't even linked suit environments before makes her a representative of this aspect of quarian culture.
So in this instance, I think her "experience" not only makes sense, but augments the exploration of quarian culture.
With that being said, it also imbues the romance with what I believe to be a key component of any romance, which is identifying the loneliness in a character and erasing it for them. For a quarian, I think this takes on a little extra meaning. It seems to me that this sense of loneliness is what a player wants to conquer, and perhaps not really virginity.