Deal you can leave the manga if that happens I'd be upset too, mainly because lily is more like a mother to Asta so that is just WRONG if they marry 
Considering that she is not only much older than Yuno and Asta (she's been their mother figure since they were little children), but she has made it vehemently clear that she isn't interested in reciprocating Asta's advances(if the water punch to the face didn't make that obvious enough back in Chapter 1), you guys are getting needlessly worked up over unsubstantiated possibilities again. This is Tabata once again giving some screen-time to develop minor characters that haven't appeared for a long time so as to avoid neglecting them(unlike a certain other author, who seemed to forget half of his characters or just didn't know how to make them relevant). Furthermore, Tabata already named Noelle as Asta's love interest back in the September interview featured on Reddit last year.
And Lily wouldn't leave her vocation when she has the other orphans to take care of, anyway.
The most Lily will probably do is note how both Yuno and Asta have grown since she last saw them, which will be humorously followed by Asta asking her out again, then getting nailed with another flat rejection(followed by another water punch to the face). There's no reason to assume that he'll be pulling a Kishimoto when, taking the entire manga into context, we've seen that he's done his best to avoid that.
Edited by Kagomaru, 26 May 2018 - 11:38 PM.