Well, the MC already looks quite feminine before and with the boobs, well I think the tag is justified. And I mean, the male protagonist is there as well.
Effeminate looking guy does not equal trans though. If it did, then Eric's futa/trans too.
Despite some people's
preferences Krowly is an effeminate bisexual man. At "worst" he's a bisexual twink who can wear women's lingerie. Unless/until Boobsgames has an event where Krowly changes
permanently to futa/trans and everyone addresses him as a woman, he's still a guy.
I don't know if some sort of transformation tag exist, that would fit here as well.
Transformation tag does exist, but I'm honestly not sure it really applies.
Suffice to say it's more applicable than futa/trans protagonist.