They could make love all they want; there is no real evidence of interbreeding or of mixed offspring other than the one freak skeleton which could be a birth defect or an indicatation that the one or two children ever born of such unions never lived any longer than that.
The problem is that given the proximity of the two groups over long time spans, there should be LOTS AND LOTS of evidence of interbreeding and mixed skeletons of all ages and, as usual, all they can ever find is the one freak skeleton and they want to claim a victory for evolution on that basis.
Likewise, I could show you a skeleton of a snake with two heads. That does not mean that all snakes of that generation of snakes had two heads, or that two headed snakes were common then, or that two headed snakes are terribly viable.
There are not LOTS AND LOTS of skeletons of anything, especially human.