I used to be absolutely disgusted by dickgirls. For years I would deliberately skip saving pages from doujinshi I found if it had one in it. More recently though, I've accepted that there is futanari art that turns me on. I don't feel compelled to seek out pictures of real-life shemales; it's just one of those things, like piss-drinking and genital piercings, that anime makes arousing even though you'd never go for it in real life. I think it's a combination of three things:
1) Attaching a PENIS to a girl causes her to become sexually aggressive. She can't stop touching herself or trying to have sex with every girl, boy or other dickgirl she sees. An erection means "I'm horny, I want to have sex." When you see a girl gazing at you with a hard on, you know she wants you. Normal girls, it isn't always that obvious (a wet pussy is hard to see through clothes for example).
2) Seeing a dickgirl ejaculate is like how you wince when you see someone else get kicked in the nuts; it's a sympathetic reaction, like psychosomatic or something. Beautiful girls and ejaculating are high on most guys' lists of favorite things; dickgirls put them together in a simple package.
3) A dickgirl's dick is just another toy for you to play with. I'd treat it like an extra breast or a really big clit, something that will make her feel good when you touch her, and a hell of a lot more user friendly than a teeny tiny little knob at the top of her crotch.
4) After four years of nothing but 'normal' hentai (and some real life sex), I'm jaded by hentai. I need something a little kinkier to get me interested, and this fits the bill nicely without getting too fucked up like scat or mutilation or sensory deprivation. GET