Busted! ;) Your dad probably wanted to clean up any evidence, but your arrival required plan B: hope you were putting your panties in the laundry anyway.
Approaching him about it would probably trigger a wall of embarrassment. He might regularly enjoy your worn panties, but never want it to be known. So to get him to leave his hot cum in your panties could be tricky... one of the only ways might be similar interruption (though he might even be shy of quickly slipping into the bathroom before you're finished now!). But if you repeat a similar situation a few times, he might not worry about leaving cum in your panties if he never gets called on it -- thinking you don't notice anyway.
If I had a daughter, I'd probably be unable to resist the lure of her worn panties. It's a fetish that can really get you in trouble. :P I had a roommate who I think realized my fetish and made it easier for me, leaving panties obvious, and even taunting sometimes by talking about new panties she bought for no particular reason. But even that was enough to make me shy of acting on my compulsion -- she might know! I share this to maybe give you insight into what might be your dad's perspective. Maybe you can think of a way to become comfortable/confident about leaving a mess in your panties. Good luck!
