This is just fluid produced by your penis to lubricate it. The
penis, in response to sexual stimulation, will secrete fluid to
lubricate your penis to allow smoother penetration during sex.
However, your penis doesn't know when you are penetrating, it will
just secrete. If I don't remember wrongly, I believe it is the
bulbourethral gland that is secreting the fluid.
Therefore, the point is that it is normal to secretethe fluid upon
stimulation, either through physical contact or thoughts.
But you said that your underwear gets all wet and there will be a
wet spot, so it may be hypersecretion. I'm not too sure. Maybe you
can just wait a while more to observe further before seeking
medical help.
