Coming out is an incredibly personal thing and it can come with some horrible consequences. It should be up to her whether or not it's worth it. Even if she never wants to come out, don't do it for her. Respect her right to be herself however she so chooses. You can encourage her and be a out and proud ally without forcing her to come out though. Of course, making out with a girl doesn't necessarily make her a lesbian. She might be bi or she might have just wanted to try it out. Doesn't necessarily mean there's any closet to hop out of. Talk to her without pressure or threats and you might find out she is being honest with herself.