why does semen smell the way it does?

i have noticed semen has two scents.

the first scent is the ocean breeze scent that you smell when you first ejaculate. sometimes it smells like bleach too

the second is when its been sitting around for a while.. when it begins to smell like squid or rotting seafood.

i have no problem with the first kind, but the second one is truly annoying.
why does semen start to smell like that, and how can i prevent it or reduce the stench?

I'm a girl, and I've always thought cum smells exactly like wet towels- like when you go swimming and leave a wet towel at the bottom of your bag for a few days.

And yeah, it tastes pretty much the same as flem.