Hi Daniel,
Yes, the walls of the anal canal and colon are thin and will absorb the semen. You probably will not need a plug but have some seepage in your underwear. A partner with a very long cock with shoot his baby batter deep into your colon and you may not have seepage. A woman's vagina has much thicker walls so semen will not be absorbed and will reach their target destination, an egg in the uterus. Here is an interesting German video I came across. Almost scientific!
shemale anatomy - XVIDEOS.COM.mp4