Say that word 'romance' over and over again. Think of all the things you associate with it, then think of Frank Castle.
For me the words 'romance' and 'Frank Castle' cant even appear in the same sentance. I mean, what would they do? What would they say to each other? Elektra at least has some evidence of being a normal person capible of having normal human feelings, but Frank is entirely incapible of that kind of involvement with another person.
As for a kid - another reoccurring theme about Frank is that he knows that he is technically the bad guy, and a monster, and traditionally he has always advocated not following the path he walks to people he would consider 'good'. I honestly cant imagine Frank ever raising a child to be a killer, it just doesnt fit on any level.
True, romance and the Punisher don't mix. Of course they could have it where Elektra carries Frank's kid but doesn't tell Frank she's pregnant and they part ways. She raises the kid and when Frank perishes he has an heir to the skull.