I think the people are not only concerned for your daughter but lets look at that verse again........
2 Corinthians 6:14-16(KJV)
14 Be ye not unequally yoked together with unbelievers: for what fellowship hath righteousness with unrighteousness? and what communion hath light with darkness?
15 And what concord hath Christ with Belial? or what part hath he that believeth with an infidel?
16 And what agreement hath the temple of God with idols? for ye are the temple of the living God; as God hath said, I will dwell in them, and walk in them; and I will be their God, and they shall be my people.
You see, actually Gods Words are not really suggestions but commands but most people of God either forget that or take it as not important or whatever the case may be but I take Him seriously. The thing is you are talking about your daughter, not you. I do not know anything about her walk with the Lord, I would think she surely doesnt know or understand the Word of God. Even though this young man is not a Muslim he is still an unbeliever as of right now and the above verses are speaking about any unbeliever. You truly love your daughter and we can all tell but at this point, since the relationship has gotten this far Dad, you are going to have to keep her in prayer, I know I would be concerned. The good news is God is able to handle this and He will. May God watch over everyone of you and protect you all and may we hear before long that this young man is now a Christian.