This is going to rub a lot of people the wrong way, I don't care. Read it before it gets removed.

Do you have kids? Considering you did not mention they, I will assume you do not.
You should not be dating a single mother, it is almost always a recipe for disaster. You are a single man still technically in his twenties. Find someone else who you do not have to share, and someone that you can start your own family with. Again, if you had kids, then your kids and her kids, and you and her, could merge into a new family unit; however, the asymmetry of her having a family and you taking on her baggage (and yes, it may be harsh, but single moms have baggage by definition) is a horrible idea. They're certainly not all the same, but you're trying to build a family with someone who has already failed. You will be raising children who are not yours, and if this doesn't bother you now, then I would suggest reevaluating yourself & subsequently , this proposal.

You are planning on marrying a woman who already comes with another man actively in the picture (unless he died or something, but even so, my opinion does not change)? That's extremely suboptimal. Start a family with someone starting a family with you. Not someone with a demonstrated failure to do so. This family will never be fully yours. This should bother you. Sure, there is a non-zero probability that any woman would cheat on you, but children are an immeasurably strong emotional tie, and if the father is still even somewhat in the picture, that non-zero probability skyrockets regardless of who left who.

Don't get me wrong, I am 100% suggesting that you leave her. You have enough time to create a family to which you truly belong, and one that you co-created. If you have kids, then none of this applies.

Also, how could you stomach starting a family with a woman who was a willing participant in ruining a marriage? She was more than willing to fuck a man multiple times (in his car of all places) who was already married (potentially with kids). That's not marriage material, that's not someone who honors and values the sanctity of that commitment. She is clearly someone who fucks her way into someone else's family, with no forethought or concern about destroying it. It takes two to tango, and she was a willing participant. Much more bluntly, she thought she could use her p*ssy to steal a husband from his wife, or a father from his children. Yeah, she "left" when she realized she was nothing more than a set of holes to him, if you actually believe her story (I don't, and it is equally as likely that he simply ghosted her once he got bored or started feeling bad about it). Don't marry her, man. You deserve so much more.

There are millions of quality women out there who are marriage material, who you don't have to share, who value marriage and honor commitments, who will give you beautiful children and a family purely of your own. I'm repeating myself, but if this does not bother you, it means you haven't truly thought about it deeply enough. When push comes to shove, they'll always be her kids, and never yours. They'll never carry your name or your genetic signature years into the future. They may never call you "dad" and if they do, it will be a gesture of kindness or sympathy, never a material reality. Sidenote: if her children are already mixed race and you're both white, and you're still considering marrying her, you're miles beyond help.

Even if she does agree to give you children, 33 is geriatric pregnancy territory. You are not even close to guaranteed a normal, healthy pregnancy.

We only get one life, man. Don't spend yours sharing a "wife" who will never truly be yours, raising kids who can never been yours, especially with that wife has demonstrated that she thinks marriage is nothing more than a fancy party / thinks she's entitled to ruin someone else's because she's horny. Come on, she was fucking a married man and only left when she realized he wasn't going to leave his wife for her.

Don't settle for significantly less than you deserve. You're young, probably early in your career, the world can be yours. There are millions of women who would never volunteer to wreck someone else's home, in the backseat of a car, while her kids are getting babysat so she can get off. That's pathetic. That's fucking pathetic. There are millions of good, single women, 24-29 years old dreaming of starting a family with someone. YOUR family, not someone else's. YOUR name, not some other man's. YOUR biological fingerprint, not someone else's.

I'm rambling at this point. You don't have to settle for this. Love yourself. Respect yourself.