In fact, without building up relationships with myriad characters in the game, you're going to miss out on a lot of what the game intends for you to see.
There are three other girls that Niko can meet and date via the
During the course of the whole game, a total of 11 men and 12 women have profiles up on that site.
While it is possible to select "Date" on any of their profiles, only two of the nine women are actually datable.
If you want to date girls other than Michelle and Kate, you can do that, but it involves some legwork.
You're going to have to go to an internet café and browse the in-game websites Love Meet and Craplist (which are always on your homepage when you logon).This is a list of in-game websites accessible on the Internet in Grand Theft Auto IV, The Lost and Damned, and The Ballad of Gay Tony.