The original story I heard was that the director originally wanted her to be naked in this scene, but she felt it wasn't credible. I didn't know about the sex scene with Skerrit. I wonder whether the director that scene cut because it took some of the tension out of the film before everyone started being killed? I must admit this "panty" scene was one of the first that I recall "jacking off" to, and then her scenes in "Half Moon Street". To be fair, I've always preferred "real" looking women. Sigourney reminded me of a couple of female teachers who made me feel horny in my teenage years.
The word on her is she has no issues with nudity/sex scenes. She prefers it, in the first alien's she was pissed because her sex scene with Tom Skerrit was cut out of the film.
gotta love em