"anal birth" is a tag this could be confused with (and "anal birth" is mainly used for female characters that birth from their anus thanks to eggs and not a womb). Even in yaoi settings I feel that it's often a roleplay thing where the character doesn't actually get pregnant. So to keep it more consistent I think it would be better to call it "male womb" or "anal womb", but when we already have "male pregnancy" in the proposed tags, is there even a point to having both? For futanari I think it's unheard of to have an anal womb