Ok, looking at the art work for the X-Legs, they look like they are from the waist down. One of my groups characters has recently gotten himself a set and it cropped up by another player if he still has the ability to produce children... So would a half orc who has X-Legs be considered Sterile?
There is no rule based answer to this question, so by the rules, a character with xlegs can reproduce just fine.
As a GM... be careful and er on the side of letting things happen, whether because they have a fridge full of samples, or because things still work, either is fine.
I'm pretty sure anyone selling that product managed to find a way to avoid that or they wouldn't be in business.
Unless your PC's species has their reproductive organs in their legs (looking at you, Star Trek VI), they should be fine.