It becomes even more so the older you get, too.

Sometimes, for various reasons -- stress, illness, feeling tired, having something else on your mind, hormonal shifts, dietary or excercise changes, you name it -- it's just not going to happen, just like your girlfriend won't always have her clitoris get hard or her vagina lubricate even though she's liking being with you, wanting to sexually perform.

And that has zippo to do with sexual orientation or love. That's just basic physiology.

So, nothing's wrong with you: you're human, not a machine.

There's also no reason for her to be disapointed. Not only is intercourse or penile sex but one of MANY kinds of sex -- many of which are often more satisfying to most women than intercourse alone -- your erection shouldn't be seen as some sort of unit of measurement (no pun intended) for her sex appeal.