If the semen (cum) slides down into your vagina opening then you can become impregnated very easily.
If your partner accidentally enters your vagina during the sex then he can push a little pre-ejaculatory (precum) fluid into you. This is also the case if the sexual play began with rubbing his genitals against yours or if it ends in that manner.
Cum is a very luquidy substance. It can slide around easily or drip into places you don't want it to go without you even realizing it at first.
Always
Always
ALWAYS
Use a condom.
Condoms aren't 100% effective on preventing pregnancy or STD/STI transmission but they still help lower the risks.
Be smart, be safe. If you don't want a baby then do everything you can so you don't wind up in a sticky situation. Have your partner wear condoms if you two are going to have some sexual play. Use hormonal birth control pills too. Set some rules on when you two engage in anal sex--like no entering the vagina at all after you've entered the anus or no entering ANYTHING without a condom on first.
It will hurt for awhile if your partner didn't use lubricant or things got a little rough. Pick up some lube from the store for the next time. Take things slower. The rubbing motions can severely irritate the lining of the anal opening and even cause some bleeding. Personally, I don't think it's worth the risk or possibly having to be rushed to the ER because things got a little rough in that extremely sensitive place. That's just me though. [ Peeps's advice column | Ask Peeps A Question ]
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