I would like to address an issue that I have raised in the past: What if Trevor is not Lilly's father?
What we know about Trevor:
He met Shannon at the burger joint at some point and dated and married her sometime later.
He bailed sometime after Lilly was born (and had enough time to "fixate" on him, which caused problems for Frank later), but we don't know the reason why. The most likely possibilities are:
1. he thought that Lilly was his daughter, but then found out that this is not so and therefore had no reason to stay.
2. Shannon scared him away.
Unlikely possibilities:
1. he didn't see himself as a father and felt constricted.
2. he had an affair and ran off with it.
Why these theories are unlikely: Shannon. She is a person who is vindictive and star struck like a mule. I find it hard to imagine that she would forgive Trevor for something he did and then start an affair with him. But then I thought to myself: she also wanted to get back at Frank at that time, because of Abby and Jessica. So it would be entirely possible that the unlikely possibility #1 would be quite possible.
I have to admit, though, that I like theory #1 the best, because it gives us a scenario of two marriages that go similarly, but have a completely different twist.
Jess and Frank, Jess pregnant by a guy who, according to our current state, made fun of her and started rumors about her, marries her best friend from teenage days and has a harmonious and loving marriage with him until her much too soon passing.
Shannon and Trevor, Shannon very likely already the mother of a daughter meets Trevor through her job and he seems to have no problem with it. Both have a daughter together later, who is totally fixated on him, which then fails because he leaves for unknown reasons and he sees no reason to ever contact his presumed daughter again.
Let's ignore Trevor and Shannon's affair for a moment. I honestly don't know what to make of Trevor and we're probably missing some information, like more details about Shannon's parents.
What works in his favor is that he married Shannon, even though she probably already had a daughter at that point.
What speaks against him: He abandoned his wife and children for unknown reasons and ran away. Again, we don't know the reason for this. In the "best" case, Lilly is indeed a cuckoo child, and while he may not have had a problem with Becca, he did have a problem with having a child in front of him that he thought was his most of the time, and as I said, he never bothered to have contact with his daughter, even after he had contact with Shannon again. Worst case scenario is this: He dated Shannon for a while because sex with her was good. That she had a daughter wasn't a big problem, it was possibly never going to be serious anyway, so who cares. Then she got pregnant and there was a shotgun wedding coming up. He played along with the family idyll for a few years, but at some point he couldn't anymore. He wasn't the type for serious commitment and responsibility and then bailed.