This sucks, I'm so sorry for what you must be feeling and going through right now. Emotions + pregnancy hormones are no joke.
If it turns out he did stray from your partnership - I'd like to put in a plug for staying calm, and seeing whether there's a way you two can make it through this. Infidelity is by no means uncommon, and doesn't mean that your partner doesn't love you or want to be with you. There's such a range of emotion and need - and while it's unfair that he might have made a choice that affects you so much (not to mention the irresponsibility of contracting an STI) without your consult or consent -- it doesn't have to be the end of what seems like a strong relationship, otherwise.
If you approach the conversation with emotion, but also with a sincere desire to understand, and not with rage (very hard!!! but doable), you'll probably have better luck than flipping out on him.
Check out Esther Perel's work on the subject. She's a very well known and respected relationship counselor/psychologist. Her TED talk. Her book on infidelity. Her (amazing) podcast.
Good luck. Sending hugs!!
I think a lot of these comments are off the mark. Here's a different perspective.
You and your wife got married very young, OP. Let me tell you something about marrying your "first and only" - even if everything is going blissfully, your mind will always wonder about what else is out there, because you simply have no way of knowing otherwise. You can (and likely will) have these feelings even if you love and respect your partner. You can't reason them away, but you can control what you do - or don't do - with them.
Society teaches us to idealize marrying your first love as a purer love, but on this point society is full of bullcrap. Experience helps us know what we want and need from a partner, and how to communicate those wants and needs. It sounds like you and your wife haven't been on the same page for a while - she couldn't explain what she wanted in a way that you were able to hear. She's been afraid to reach out to you more directly, so has tried to explore her needs in other spaces - which is not good - but has still tried to respect her commitment to you by not crossing the "big" boundary of sex.
But I suspect that this is a case where her "infidelity" is not about feelings for another guy, or a desire to roam - it's about what she hasn't been able to find in your relationship. I wouldn't just write this off as "she cheated, dump her."
Therapy is definitely a good idea, and I'm glad that you're creating space and boundaries to help both of you process this. You might want to look into some work by the couples' therapist Esther Perel. Her podcast "Where Should We Begin" (https://whereshouldwebegin.estherperel.com) has live one-shot therapy sessions featuring couples who are facing big issues - sometimes infidelity - and she is very good at exploring the bigger picture as well as perspectives of both parties. She has a lot of insights that can apply across many relationships. You may also want to look at some of her books, particularly The State of Affairs (https://www.amazon.com/State-Affairs-Rethinking-Infidelity/dp/0062322591/ref=pd_lpo_1?pd_rd_i=0062322591&psc=1) or Mating in Captivity (https://www.amazon.com/Mating-Captivity-Unlocking-Erotic-Intelligence/dp/0060753641).
I don't blame you for feeling bewildered and betrayed right now. It's immensely difficult to look through another person's private emotions, writ large. Only you get to decide whether you think you can trust her again, and what regaining that trust might look like. If you can't ever do that, that's okay, but you sound like you would like to do some exploration first. Good luck to you.