I wanted to try to rationalize everything, until the last sentence of the second paragraph. It's even possible nothing happened beyond drinking and hanging out with them considering how she worded that message. But it's still a problem that she lied to you.

So where you go from here is you talk to her. I'm the first person to come on here and say I don't condone snooping. But what's done is done. You did and you saw what you saw. The fact that you snooped doesn't make reality any less true. So you can either keep pressing the issue until she opens up, or you can just be honest that you saw the texts. As it stands, she's trickle truthing you.

She'll likely turn things around on you for snooping, but again, that doesn't change reality. So hold firm and talk this through. I don't want to sit here and just jump to the extreme and say she's cheating and you should leave her. I don't know if she's cheating (and right now I honestly assume she isn't), and I don't want to jump to ending things because if no cheating occurred, if she just comes clean, this is something you should be able to work through, and then of course you have a child together.

I would, however, suggest you don't rush to get married until this is all sorted out. Because if ultimately you decide you can't trust her, then at that point you shouldn't be in the relationship, let alone consider making it legal. Good luck.