Well, gee, I haven't had sex since I've had lupus.
However, as a nurse (disabled), and as a lupus patient, I can tell you that the autonomic effects of sex could definitely induce a flare. Your heart is racing, parts of you are flushing and having all sorts of reactions, you're breathing hard... what lupoid body in its right mind would not have something to say the next day?
Imagine sex as a workout. Now, you see the connection?
How to deal with that emotionally and relationally, I shall leave to those more in tune with that.