It's called "painting" where I come from and it is an expected part of "heat of the moment" sex. If you or your partner using the strap-on have any issues with this, slip on a condom. Any mess is easily removed.
If your are going to be with someone who is planning to use the natural stuff in your bottom - USE A CONDOM 100% of the time!!!!! No exceptions until you both trust the other with your lives. You avoid any painting amongst more permanent issues.
And read post #9 here :
http://forum.transladyboy.com/showthread.php?t=360