On one hand, informed consent seems to be the rule for interpersonal couplings. On the other hand we still get squicky about incest, even when accidental offspring are not a concern (say, a gay coupling, or the sister is barren).

Centaurs are notoriously randy, and I suspect would take to horses like shepards to sheep if they were lonely enough. But yeah, the genes might be incompatable, or create a non-true offspring like mules and Labradoodles