That was an oversight on my part, I assumed that if you kept Wilson on campus it's because you were interested in her, but I can see some edge cases like you described where that isn't the case. The next public release will include a "let her move in by herself" option for people that are stuck with her. In the meantime, if you know how to edit your save/variables, you can set $wilson_stay to "m", "p", or "g" to skip over the event.
EDIT: Just to answer your initial question though -- you can avoid having sex in that scenario if you used the permanent purple light on Patel to go to her house (you can then have sex with Patel instead) or if you convinced Ms. Hill to stay on campus, you could have sex with her instead (she moves in to the Motel/Patel's house with Mrs. Wilson if you kept her as well). And finally, some of the Monica Friday night climax resolutions offer the option to banish Mrs. Wilson/convince her to leave so you never have to deal with her again (I assumed players that felt this strongly about her would do this option).