idk about marrying but I'm in my early 40s and have long term dated a few mothers, some with kids in their late teens. In short, it entails responsibility and tact.

Being a recurring kinda-parental / adult figure in a kid's life has an impact on their notion of what adults do, don't do and what kind of adult they aspire to be. This entails responsibility. Forming any kind of meaningful bond then having it go away has an impact on anyone, but on children it can be harsher. If you have a heart you don't mess around with a mother that you're not really interested in and you should only meet their kids if you're serious in wanting something stable/long term.

Basically It entails a lot of responsibility that you can either assume or risk making not only the mother's, but some poor kids' lives harder. If you don't care about any of that, that's up to you (but also, you're an asshole).

Lots of wonderful women are mothers, though, and many are more than worth the extra care.