It is a joke but to be fair, Solas does behave like a bad boy quite a lot.
1) He puts you to sleep without your consent and the proceeds to seduce you (and possibly f*ck you) in your dreams. I find it ironic how no one considers this as a form of sexual assault or violation of the mind given that we live in a time where the cultural narrative says that women cannot give consent when they are inebriated. Logically, Solas be seen as a sex offender or a privacy offender.
2) He exhibits a general attitude of aloofness and indifference which bad boys do exhibit. He even says that bedding you is a side benefit of his attitude. I am surprised at the sheer number of female Lavellans that want to be treated as a side benefit.
3) He breaks up with you but he stalks you in your dreams. I find it weird how people, especially women, do not consider this a form of harassment. Then again, women are perfectly okay with Edward Cullen and Christian Grey, who do the stalking thing as well.
If you want actual reasons instead of humor, here is one:-
1) Are you referring to the scene in the Fade? There is nothing in the scene, to my knowledge, that indicates that the Inquisitor's mental faculties were impaired with the exception of how they percieved the Fade as reality. As a note, Solas' reaction to their surprise at this revelation and previous wording suggests he thought they knew they were in the Fade; "Where do you think we were?"
It is the Inquisitor who has to initiate the kiss for the romance to occur and the kiss is all that happens in that scene. There is no possibility that sex occured as the Inquisitor wakes up immediatley after Solas wakes them up at the end of the scene. There is no fade-to-black during the Fade portion of the scene and nothing, as far as I can tell, that suggests something sexual did happen.
2) That's... not how I see the optional dialogue bit. It also seems a bit joking, to me. Here's the dialogue for those curious;
Solas: "Forgive my melancholy. Corypheus has cost us much. The Temple of Mythal did not deserve such a fate. The orb he carries, and its stolen power... that, at least, we may still recover. With luck, some of the past may yet survive."
Inquisitor: "You're being grim and fatalistic in the hope of getting me into bed, aren't you?"
Solas: "I am grim and fatalistic, getting you into bed is just an enjoyable side benefit."
Although I think the interpretation of the relationship as having a sexual component has been retconned away to be honest since he says this in Trespasser when a certain option is selected;
Inquisitor: "And so he did."
Solas: "I did not. I would not lay with you under false pretenses."
3) I admit I am not fond of that bit because that is a pet peeve of mine to see but since I believe it is open for interpretation as to why he does it and how the Inquisitor feels about it, I can tolerate it. I know others are likewise not fond of it.