The sphincter needs to be coaxed to relax since it's job is to keep stuff in until the right time. If you can slip a dilio in the bottom without any prep time, there are probably going to be some issues later.
First realize that dildos and real life cock are not the same. the real thing has more give than a dildo, so it's actually tougher to insert a dildo. One thing to do is to massage the pucker with your fingers first while masturbating. the combination of arousal and stimulation of the prostate and sphincter will loosen you up. A little finger-fucking will go a long way - trust me. For goodness sakes, also remember to remove hang-nails or sharp nail edges!!!
With a partner, rimming is an option. There are some health and cleanliness issues around this (see my posts on enemas), so you or your partner may not want to do this. Goodness knows I keep mine sparkling clean and don't get that kinda booty lovin' (grumbles).
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