AFAIK there isn’t any evolutionary pressure for it to be any particular color and its content doesn’t have anything to make it any color other than more or less whitish.
According to the dipshit who assessed my load at the Family Planning Clinic here when I was preparing for marriage - and I was young then - my love juice was classified as “greyish”. I’ve never forgiven the guy, even though my wife has tried to convince me that he was driven by a sense of inadequacy and had been taken in by local folklore, which puts it about that Western jism is more powerful than local varieties.
A big, big thank you for giving me the chance to get this off my chest.
Not that it usually ends up there. More likely the ceiling.
There are a lot of white-looking substances made of heterogeneous mixtures of colorless, transparent phases. Clouds, semen, mayonnaise and white paint, for example.
If you have a heterogeneous mixture of phases, it means there are millions of little regions of the substance that don’t mix with one another. For example, a cloud consists of transparent air with millions of tiny transparent water droplets suspended in it. The water droplets are a separate phase from the air (meaning they have a different compostion), and have a different refractive index from the air (because they have a different composition), so every time a ray of light passes from the air into a water droplet or vice versa it bends some. Any given ray of light that hits a cloud gets bent in a sort-of random direction many times, which causes the cloud to scatter most of the light that hits it, more or less at random and without too much regard for its color. A ray of light bounces around inside the cloud until it emerges again into the dry air, usually on the same side of the cloud that it entered from. That’s what makes a cloud look white and opaque.
With mayonnaise it’s the same thing, except you have a transparent oily phase mixed heterogeneously with a watery phase (with a different refractive index). With white paint it’s the same thing, except you have transparent pigment particles with a high refractive index mixed with a transparent binder phase of a lower refractive index.
In the case of semen, the carrier is mostly water, but it has lots of sperm cells (and other phases, I think) mixed in with it, and the same thing happens. Light rays going through the water pass into and out of sperm cells (and other phases) and are deflected at some odd angle each time because of the difference in refractive index (which is caused by the difference in composition between the water outside and the cell membrane and other mostly colorless and transparent components of the sperm cell). Any given light ray bounces around from water to cell to water to cell until it comes out again at some random angle, usually on the same side of the semen wad that it entered from. The net effect is white-looking, semi-opaque semen.
Please note that what asterion said is incomplete. You can mix together lots and lots of organic substances that don’t absorb in the visible, and as long as they form a continuous phase (which depends on what you’ve mixed together), they’ll look colorless and transparent. “Whiteness” comes about because of phase heterogeneity and differences in refractive index between different phases.