I can understand what you are going through. Being a bi sexual is not a very rare phenomenon and there are plenty of people who face it. The fact you are getting married to a girl clarifies that you want to continue in the society as a hetrosexual. Infact its difficult to completely differentiate homosexuality
and hetrosexuality as it a form of continuum. So please dont bother yourself much with this issue especially when homosexuality has been legally accepted. Secondly coming to your problem about the sexual act, you appear to be suffering from permature ejaculation as you ejaculate before you can start your sexual act and the later on the erection disappears. Erection is bond to disappear after the ejaculation. You need to indulge in more of foreplay than proceeding right away to the sexual act.There are some drugs like Paroxetine
available in the market which prolong your sexual act period and may help you.
Hope I am able to solve your concerns.
Thanks . have a nice day!