Such an intriguing line!
Maybe when he goes into mateform, he's attracted to other males, which makes other listners uncomfortable because "uhh... you're not going to make a baby that way bud!" Maybe after trying once he just decided somehow he was a weird case and shouldn't bother again.
I wondered if it could be possible for him to be trans (ie, turning into female mateform instead of male), but there's really nothing else to suggest a) that Rlain has a femalen identitiy outside of mateform, and also b ) until recently he wouldn't have had access to the kind of bond that does large scale physiological changes to match identity.