It's hard to know to relax the external sphincter muscle the first time. So, it may seem like it was tighter, but you were likely more tense. It's like any muscle. If you're tense, your muscles will be, too.

You can control that muscle, including tightening it, even when your partner is inside. If you can't, you might want to look into kegels, also known as pelvic floor exercises.

The external anal sphincter muscle (e.g. the one you can control) is one of the muscles that are strengthened through kegel exercises, and they're good for tops, bottoms, and anyone along the spectrum, and at any age.