I think for a couple of reasons.
1) Because her and Elena are close friends, almost like family. She would hope that Elena would be happy for her and support her. Any true friend would want you to be happy about your decision. (major reason)
2) She has no other female she can turn to. (minor reason)
That we did, and I'm thinking it's because of that close bond they share. Though Elena isn't really thinking about that aspect. She just wants to "try" D now that Elena has experienced a little girl-girl play with Jennifer. I think that was just a little experiment and is wanting more.
If you saw the last update of Babysitter,then you know things can go not the way we want.Jessica surely don't like the uncle/niece relationship(hopefully we can change that in the future),not to mention she obviously in love with Christine.