I got an erection while making out but no hard on while watching porn

It sounds psychological, and it's far more important that you get hard when you're with your gf than watching porn.  How old are you?

Guess it goes to show how much better the real thing can be. My guess is that if you relax and let your girlfriend tell you what she wants, you will find that penetrative sex is less important to her than you think. Remember, too, that caressing her body with your hand, lips, tongue and fingers are often much more likely to pleasure a woman to orgasm than vigorous thrusting, though hopefully you'll both enjoy that too. Anxiety is the enemy of pleasure. I always found that taking things slowly and concentrating on my wife's needs allowed my body to respond better than worrying whether I could 'perform'. Mostly - unless you are very young and can come twice without a break - it's often best to help her reach orgasm before you enter her. Most women can come several times without the need to rest like us (older) men between climaxes. So you would have plenty of time to allow your lovemaking to give your penis a chance to stand to attention. Also I take the view that it's as much the woman's responsibility to make things work as it is for the man to get it right. It's different for every couple and often different each time. Sex is often a very hit and miss clumsy affair. Keep your sense of humour and it'll be fun anyway!

Best thing I can suggest is to talk with your girlfriend about it.