It's both greed and need. What makes us cumsluts is an insatiable need, and knowing that the loads are being shared just runs up against our nature. i can remember when i was 8 years old, my mom gave me a birthday party and had a gift for every kid that attended; gave it to the kid as he was leaving. i was outraged. It was my birthday -- all the gifts were supposed to be for me. Being a cumdump at a gang bang is pretty much the same thing. From my perspective, the whole purpose of the party is for me to take loads.
Now, going to a house party is different. i go with a date, and if He ends up fucking me, that's totally cool. And if other guys at the party are also being fucked by their dates, that's cool too -- even enhances the party. But if other guys' dates want to fuck me when they are through fucking their dates, that's even cooler. i guess it depends on your mindset going in (but my mind is ALWAYS ready to change from date to cumslut; i can turn on a dime)