A couple of issues there. I've been filling out applications for the past 9 months straight and I have never seen anything about sexual orientation. The closest I've seen is Sex: __Male __Female.
You would have to write it down as a note in the margin if you wanted to make your lesbianism known to them. The best case scenario is they ignore it, the worst case is it reflects poorly on you for wasting their time with information that's irrelevant to the job you're applying for (sure it only takes a split second to read, but I've seen a lot of employers act as if employees were carrying off a cash register just from wasting a nanosecond).
Another issue is that protection from discrimination based on sexual orientation works both ways. If you were in competition for a job with a heterosexual woman (or man, for that matter) and it was discovered that the ONLY reason you were chosen over them was because of your lesbianism it could open up the company to all sorts of litigation.
