WMG / Go Get a Roomie!

The hints at Roomie being an actual alien are just too blatant for it not to be true.

Roomie and Lilian are Fairies
Fairies in the "Roomieverse" look exactly like humans in every way but have magical powers. Roomies magical power is being near irresistible to humans, but her powers don't work on other fairies.

Lillian's dreams aren't just dreams

The series is actually Lillian meditating or dreaming

Think about it Lillian is a huge hikkikomori who like to live in her subconscious all the time rather than live in the real world. Now take that to its logical conclusion. All her dreams are actually a Dream Within a Dream. This will lead to a The Reveal where she has learned enough in her dream to actually go out into the real world. Well, either that or accept the dream world like in Eternal Sonata?

Roomie has been sexually abstinent for a month because she wants to get serious with Lillian.

Roomie's sexual promiscuity has been a reflection of her belief that every human being is deeply special and seeking a unique partner is a denial of everyone else's glory. But the thing is, even granting that all people are incredible things, some people are better fitted to each other — not in a "these two people are meant for each other" sense, but "this person makes me feel valued for what I like about myself" or "this person keeps me grounded in what I care about" or "it feels so good to make this person happy" or ... well, there are too many ways to count (none of which even require binary relationships, for that matter) — and when you read the series, you can see that Lillian and Roomie fit. They make each others' lives better in many ways, ways which I think Roomie hadn't felt before. (Be honest: how many people do you think Roomie knows who won't let Roomie use her charm to avoid talking about something?)

My hypothesis is, then, that Roomie is realizing that the idea that has been the basis of her whole lifestyle has been undermined — and that the way she feels about Lillian is so much like what the stories say about "true love" that she doesn't want to act like those feelings are the same as the feelings she has for everyone else.

A final point of evidence:
Lillian: You escaped. I'm impressed.
Roomie: Avoiding sex is hard.
Lillian: Most people would complain about the opposite problem.
Roomie: Do you complain about it?

Yes, it could be read as "think of yourself — maybe most people don't complain about that", but it could also be read as, "have you thought about you yourself being in a sexual relationship with anyone?" Which might very well be the direction that Roomie's thoughts are going.