I'd think White to White conception is even harder. Let us assume the sterility problem is either number of gametes (sperm, egg) produced or their viability. Humans generally drop one or two eggs. If Lara does not drop one, no matter how fertile her partner is, she does not get pregnant. Males may eject 50,100 MILLION sperm. If Whamps are putting out 10 K - that will cause infertility. Just ONE sperm will do it. Guess they strike out a lot though.
Let us assume the problem exists for both sexes - Thomas seemed surprised he got Justine pregnant (if HE did.. Nemesis, you know), so assume at least low male count. So if Lara is NEARLY barren, she will want to maximize the sperm available when she DOES drop an egg - get a human (assume wizards count here). Thomas would want a woman he can assume DOES drop an egg - get a human. If wizards are the same as human, a wizard might be ideal - with their longer lifespans, they likely have a longer period of fertility than vanilla humans. Don't have so swap out partners as much . (you CAN, but you do not HAVE to)
(Oh, and for the ultra woke, the above also explains why when birth control was developed they went after the one egg, rather than having to kill every sperm. Just an easier target. Add on, killing testosterone tends to kill desire... he just don't WANNA. Kinda like Vonnegut's Ethical Birth Control. If it's numb, no fun.)