Zig Zagged: Alicius sleeps with Bob, because they both seemingly love each other. It is revealed Alicius needed to drain Life Energy from Bob. Bob calls her on it. She claims she can drain Life Energy during sex, but wasn't doing it to Bob and had sex with him because she loved him, and that she can feed in other ways. Then she has sex with Charlie, and leaves him catatonic from draining.
Demons do not need to feed, or feed in other ways.