If you have ever ejaculated under water, you will know that semen becomes stringy and glue like. It doesn't seem to desolve completely on contact with water. So when it dries it seems to be slightly waterproof. I have had baths that last over 30mins and I could still smell it afterwards.

I don't have a weird metabolism or any UTIs, this smell only occurs if I forget to wash the semen off my dick and don't wash for say 6 or more hours.