Whenever they gather cum in those large, transparent trays or cups and gulp it down... is that real? Cause that's pretty gross.
If you mean are AV idols willing to drink semen? Then yes in general most AV idols are not very hesitant to drink cum. Pretty much a girl who think cum is gross would not enter the industry. I'm even pretty convinced that agents and studios would warn all newcomers what other yucky things they have to endure otherwise they wouldn't bother to sign the girl.
If you mean every time a large glass of goo is being glupped down can you believe all of it is semen? Then... well I'm not fan of that genre so I won't be too strident about it, but let's just say, in AV, as in western porn and as in cinema/tv/entertainment in general, what you see, in general, can't be taken as what you get.
I tend to doubt these not for the girls, but for the guys. Getting 120 men, do you give them private rooms? (doubt it) girl mags (yes) fluffers (no, I'd guess). 100 guys, in a less-than-ideal environment, with no hot girl in sight. With 10 guys it's not hard at least for each of them to visually see the girl who promise to drink your sperm, but 100 guys, it has to be in your head only, plus the girl mag. Speaking for myself, I doubt I will produce a full load in such as situation. Call me a doubting thomas, when it comes to collecting a pint, a jug of semen in a reasonably cost/effort efficient way.
You must not be familiar with Milky-Cat's videos
Not just a cup but a pint.
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They gather up over 120 guys for these videos and they show the whole collection process. Sure there's room for camera trickery but I find it hard to believe that they milked that many guys just to use fake cum in the end. And when the girls drink it, they have a really hard time (gagging, crying, etc) which makes it all the more likely that its real semen. They even show the behind-the-scenes where the girls throw up after, which kind of breaks the fantasy if you ask me.
Admittedly they sometimes do add some "pre-collected" semen to the final mix which I'm pretty sure is fake.
The first video is DMC-26 and the girl is Rina Hatsume (she goes by Rin Momoi in the Milky-Cat videos). Halfway through chugging the entire glass she gags and throws up a little.
The second video is DMC-25 (can't find a link right now) and the girl is Hikaru Aoyama. The video fades to black as she begins to puke the cum back out.