Trouble getting with women? Try /r/seduction and study there for a bit. They go into the science of picking up women. Also, read The Game by Neil Strauss; it is the story of how he became the world's greatest pick up artist. Great book IMHO.
Now that that's out of the way lets move on. You are right; pussy is never free in the same sense of that there's no such thing as a free lunch. Every party involved has ulterior motives whether it be getting laid, favors, or anything and everything else. However, this part is not the root of your problems.
Now on to apps like tinder. There's a common saying on /r/tinder that states in order to be successful on there you have to follow 2 simple rules.
- Be attractive
- Don't not be attractive
The problem with that Tinder is that it is purely based on looks so people will only swipe right if you are attractive, and is rarely based off of what is in your bio. I could go on forever about how to make yourself attractive and how to keep a good convo going but I'll just say go to /r/seduction.
women don't just have sex for fun
Yes they do. All the time. Talk to 100 women and a good portion of them will say that they enjoy having sex, especially with their boyfriend. Sex is pleasure and pleasure is fun; and people want fun things in their lives.
As for the entire porn argument. Some women get into porn because they love sex and they fire why not get paid for it. This is most likely the minority however. A lot of women in porn are doing it because they need the money (school debt is a big one) and they figure it is an easy job to do and it pays well. If you were desperate for money and a hot girl I'm sure doing porn would cross your mind at least more than once. And while it may bother them, they are getting paid and if they are willing to do those things for it then who are we to say otherwise?
To me personally, it sounds like you are mad at women because you aren't able to get your way with them. As if I didn't plus this subreddit enough already, try /r/seduction. It is a great community and they will help you improve your game.