I learned as an adult (he didn't tell us any earlier) that my widowed father made a conscious decision not to re-marry as he didn't want anyone to distort, corrupt or otherwise affect the memory we had of our late mother.
I was tiny at the time of bereavement, but had older siblings. I think his decision was a noble and selfless one. In later life he had one or two long and significant relationships, but still refused to marry. The last (she predeceased him by a year or so) was a marriage in all but name. But we were no longer children, which was significant. In fact two of us were parents ourselves. A re-marriage at that age would have had zero impact on us.
He (our father) was a gregarious and attractive man who enjoyed the company of women. Women also seemed to enjoy his company, so our home didn't lack the feminine touch. But there was no unnecessary complication of our young lives with the need to 'choose' between this and that adult at some level or another. I see this too often in young people whose divorced parents have re-married. It is not universal, but it is not uncommon.
I realise that breavement and divorce are entirely different circumstances, but I see my niece and nephew dealing with the uncertainty of divorce, of re-marriage (on one side) and a constant flow of women they get close to and then never see again (on the other) - and I marvel again at the self-control and selfless approach of my father.
I condemn no-one for their romantic choices, but I see among the friends of my children, my wider family and society in general many young lives that are bollocksed up beyond belief by people reproducing with partners they are unwilling to commit to in a meaningful or longstanding way.
If adults are having difficulty dealing with the ex of a current spouse, I feel a tinge of sympathy. But it is the children, who see all, hear all and feel all, who are the parties who will pay the highest price. Not only do they see it all, they are usually unable to process it. I know many young adults whose parents said they took great care that this, that, or the other aspect of divorce and re-marriage didn't affect the children... Yeah, right. That's EXACTLY what the children would say, too.
I condemn no-one in this matter, but I agree with the OP in the matter of giving VERY CAREFUL consideration to the person you have unprotected sex with or exchange vows with.
I may be very wrong and I usually am.