Re: Question about my maturing
Nothing is wrong with you at all. Note that puberty/penis growth is something that lasts until your early 20s so only being at 14 you have a lot of growing to do.
Sexual thoughts are going to happen all the time because of hormones. Now what you need to do is learn self-control. Just because you have them doesn't mean you need to be having sex.
You would know if it were urine because urine is a substance that you should be familiar with... usually when you're young it's clear so it's probably semen. Also your body naturally prevents you from urinating when having an orgasm.