You are very smart to want to discuss these things BEFORE you get married and live together.

The best way to discuss this without sounding insensitive or uncaring is to simply ask "what would you do?" questions.

Without being confrontational, ask him "what would you do if your son got in trouble with the law?" "What would you do if your son needed to move in with us? What kinds of rules would you lay down? For how long would you allow him to stay with us?"

Ask him your questions. See what his answer is. Be sure you get specifics. Your definition of "temporary" may be three months, his may be 12, or a vague "let's just see how it goes".

Be careful that he doesn't just tell you what you want to hear. The best way to encourage him to do this is by acting like you don't care what his answer is.

You are right that this scenario happens a lot and causes a lot of issues in a marriage. It's better to be sure where you both stand before getting married.
His inability to say no to his kids is a big red flag. Maybe it's better you live in separate residences for a while longer, until you feel more confident about how he would handle his kids and their issues.

My husband and I were married for a year or so when his ex called and wanted us to take the son, his wife and their baby off her hands. She couldn't stand them any longer. I was a bit upset. Luckily I was in the hospital recovering from serious surgery, the doc knew I was upset and he ask "what's wrong". I explained and he said "don't worry about it, it's not going to happen". He talked to my husband and told him we cannot have a baby in the house. He said it's to much for me, I don't need a baby climbing all over me.

Later I told my husband we can't have your kids live with us because if your son gets mad at me (bad temper) he will stay mad at me for ever. If he gets mad at his mom he will get over it. My husband understood that.

Amber's suggestion is good. You need to make it clear to your BF that while you value your relationship with him, you also value your own space and that while you understand his wish to help his kids you're not up for sharing your space with them. To that end it would be happier for you both to have separate living arrangements until the kids are truly self-sufficient.

I have a feeling these kids will never truly be "self-sufficient". If the OP is already concerned about this enough to post on an internet forum, that's not a good sign.

This is a tough one. I know you said you do not have kids, but if you did and they were having trouble, would you let them move in with you? That's pretty much how you have to look at it. Are you OK with extended holiday visits? Week or so at a time with the kids and grandkids? Or does that cause anxiety too? Talking in hypotheticals with potential husband isnt the same as reality. He may say he wouldnt want them moving in, and even mean it, and then a year later his mind can change. And even though you can say, "but when we discussed this you said they wouldnt move in", that will be a moot point if he suddenly needs to help his kids. Can you live with that possibility? As you put it - "ending up being my existence". It might not go that way, but it could. Good luck.

Thanks to all of you for your responses. They are all good advice, and something to really think about. I am leaning toward not rushing into marriage just yet, but regardless this will still be a topic we will have to discuss in great detail, when and if we ever do marry.. I see so many people just "jump into" situations and families when red flags are already up, but I knew I needed some advice on how to approach a delicate topic of children, because once you say certain things, you cannot take it back...Thanks again!!

I think you are very smart to think about this and discuss it before marriage, not after when the problem comes up. I was very naive when I married.