You say it is "highly likely you will marry this girl", is that something that the two of you have discussed or is that something you feel will happen because you do not know any different as this girl is the only girl you have been with. (I presume that this girl was your first and only girlfriend not just who you lost your virginity too).
You are at a point in your life where you see the world around you and you want to explore it, to have different sexual experiences and just to live a little. This would be normal enough.
You say you are horny but does that mean your girlfriend is not horny and or sex between the two of you is either scarce, non existent or just dull. Maybe you should try explore avenues with your girlfriend. The outcome of that could be one of two things:
1. She is just as horny as you and blows your mind letting you live a happily married life.
2. She is not as horny as you and this leaves you frustrated and causes you to visit escorts.
My advise to you is try option 1 first and if that doesn't workout then you would seriously have to consider the relationship as if you were to continue then you are only setting yourself up for trouble in the future as your needs will not go away, they are what they are, needs and needs need met no matter who does it.
Don't assume you are the only one who gives a shit about cheating on their wives or girlfriends as many people do not punt and do not visit escort websites proclaiming how good they are because they give a shit. Escorts don't care what you are doing, as has been said its a business transaction. They don't judge you but they probably just wouldn't go out with you
Anyway if you have gone to the trouble of setting up an account here you are on a slippery slope so you better put your thoughts of marriage in hold (especially at 23) and think about what you want.