Only you can know if getting married is the right choice but your letter raises some concerns. The fact that you recently slept with a man and thought it was “amazing” is problematic for several reasons. One reason is because this may indicate that you are gay. This may partly explain why the thought of marriage makes you feel ill. Heterosexual men would never sleep with a man even if their fiancé refused to have sex with them. Why? Because they are simply not attracted to men. If there are questions regarding your sexuality it may be wise to postpone the marriage until you sort out these issues.
What is also problematic for the relationship is that you cheated on your soon-to-be wife. Cheating before you’re married is not a sign of a healthy relationship.
You also mentioned that you feel that getting married means that you will not get to travel. If this is important to you and you feel that your fiancé will be holding you back from traveling, this can cause problems in the relationship. You may later come to resent her for “holding you back” from engaging in activities that you enjoy.
As I mentioned before, only you can know if you want to be married to your fiancé. It is important, however, that you get married for valid reasons and not because you feel pressured by her and her family, your family or because it would be too difficult to call off the wedding. These latter are not good reasons to get married.
Based on your letter, it seems as though you might need a little more time to know if you are making the correct decision. You need to be sure getting married is something you want and you are not doing it to please others or because you can’t say no.