It's not unusual to feel the urge to urinate during intercourse because there's pressure being placed against bladder. This seems to occur with some positions more so than others... male entering from behind is one such position. So, it stands to reason that occasionally... you might actually leak urine.
There's a similar sensation with orgasms. When you peak and release... everything just seems to "let go." And, maybe, ejaculate along with some urine are being released. So, I think that severin makes a valid point... the yellowish discharge could very well be a result of ejaculate combined with urine.
I not sure how you could stop it, though... since this could all be an involuntary reaction to sexual stimulation. Maybe, you could void your bladder by urinating just prior to intercourse... perhaps that could help bring some of the excessive wetness under control.
I don´t know but it can be just your ejaculation - the amount can vary but I am not sure about the colour. Or even if it was mixed with some amount of urine, it can have something to do with the fact that it is more difficult to control the muscles during orgasm etc, so i would not worry about it too much if it does not cause any pain/discomfort.
As a man, I would be more than happy if it was either pure ejaculation or ejaculation with some traces of urine - it´s not a problem for me.
But I am not a doctor/expert - so if you think it can be related to some health issues, it is something different.