To continue with my 2nd suggestion, I'd suggest make effects like barbs being able to cause ovulation and the flared penis able to remove pre-existing fluids base line effects for their types. Make human one vanilla option with the benefit of !breasts stroking to happen before it is erect (which I think already exists honestly). Dragon I have no idea where to start on a suggestion here for mechanical effect. I get it is ribbed so increased arousal/lust gain for partner would be good, but I also feel like it needs something not tied to stats like the others get. For knotted it would be nice if knotting your partner actually did something more than a flavor related to it. Maybe make it so the knotting as an option happens after penetration or as a second option to it with knotting penetration making it so the ability to resist is impossible or very difficult. Got a 3rd suggestion now related to this topic, but finish this thought first.
By giving the types their own effects it makes your PC better at certain kinds of sex options. It also could add some new avenues for story elements and could make strapons and dildos interesting if they had the same kind of effects. Maybe even add in new kinds of both you can load up with cum you could pump into someones. Add a pseudo impregnation feature for purely vaginal characters and tie into the new 3rd suggestion. Also for PCs with vaginas (and even asses with a certain starting perk) would have reasons to worry about or seek out certain penis types. Like avoiding barbs to not risk forced ovulation or seeking one out (Risha for example) if you don't have/want to buy heat pills. Same for knots becoming risky if it is a penetration type rather than a finish type in more non-con related scenes. Or you might want to avoid a flared penis if you are trying to get knocked up by a specific NPC and having that cum removed. Suck if you want to have Nova's pups and some random horse dicked NPC removes it all ruining your plan. Then you could make perks related to making these effects more/less effective.