Respectfully ma'am, this would not be sin on your part because you are not a Christian. The Bible, which teaches us God's will for Christians is very clear: we are not to be unequally yoked with unbelievers. The very fact that you would even consider this is evidence that you are no servant of God. You can only serve one master. Either the Lord Jesus Christ will rule over your life and you will obey Him, or you will live in open, obstinate, and flagrant rebellion and disobedience against Him. You further say that "people are good or bad no matter their faith". The Bible says this is simply false. Romans 1-3 is clear. Nobody does good outside of Christ. The one who is unregenerate cannot obey the law because they are not even able to (Rom. 8:7). You even consider whether Islam is actually true and say, "Also if that thing is true about heaven, then I’d like to be his 71st". For these reasons, ma'am, I do not believe the most urgent question you ought to be asking is whether you ought to marry this man, but rather, you ought to be asking whether you are spiritually married to the Lord Jesus Christ or not. These are hard words, yes. Harsh, even. But they are necessary. These are the most important words anyone will ever tell you. May the Holy Spirit do with them what He wills.
I leave you only with this:
Hell’s Best Kept Secret