K.B.
HI H.,
You bring up an interesting dilemma in single parenting. I decided early on (as recommended by my Psychologist) not to introduce any men at all into my daughter's life unless we were ready for marriage. But then of course, you run into the problem of finding the time to get to know the BF without the kids around. It just isn't realistic to find enough childcare for that. I am in the same situation now. On the one hand, he was a wonderful father figure (her bio dad is not in the picture at all) but now I really regret allowing them to get so close because I know that it kept me with him MUCH longer.
My co-worker is the ex-boyfriend in a similar situation. They chose to let him see the kids occasionally, birthdays etc. My co-worker is a very healthy, good man and It works out really well for them.
If (and only if) the guy is a healthy person and good male role model, I don't see the harm in letting him see the kids on occasion if he has been an important part of their lives.