Thanks for your question about the use of the unity candle. To be honest with you, I am not sure of the origins of this tradition. If it's origins are that distant and unknown, I doubt that it is worth the effort for us to pursue. I heard John Mac Arthur speak the other day and he was mentioning the number of traditional marriage traditions that come from ancient Rome. I would think that much of what we traditionally do at a wedding ceremony roots that are not necessarily biblical. (I'm not really that attached to many of the marriage traditions, personally, and certainly not the extravagence.)
I would say that in about half of the weddings I perform there is a unity candle. I don't promote the use of a unity candle, nor do I resist it. I believe that the way we do it, it is biblical and free from any pagan practices. We simply say that the couple is going to light the unity candle, and that this act symbolizes the couple coming together as one. While the candle is being lit we read Genesis 2:18 (or a bit later) through verse 24. I find it hard to see anything questionable in this.
I would think that the burden of proof rests with your pastor friend. I'm inclined to apply Paul's words here which pertain to "meat offered to idols":
10:23 "Everything is lawful," but not everything is beneficial. "Everything is lawful," but not everything builds others up. 10:24 Do not seek your own good, but the good of the other person. 10:25 Eat anything that is sold in the marketplace without questions of conscience, 10:26 for the earth is the Lord's and its abundance. 10:27 If an unbeliever invites you to dinner and you want to go, eat whatever is served without asking questions of conscience. 10:28 But if someone says to you, "This is from a sacrifice," do not eat, because of the one who told you and because of conscience- 10:29 I do not mean yours but the other person's. For why is my right being judged by another's conscience? 10:30 If I partake with thankfulness, why am I blamed for the food that I give thanks for? 10:31 So whether you eat or drink, or whatever you do, do everything for the glory of God. 10:32 Do not give offense to Jews or Greeks or to the church of God, 10:33 just as I also try to please everyone in all things. I do not seek my own benefit, but that of many, so that they may be saved. 11:1 Be imitators of me, just as I also am of Christ.
The NET Bible, (Dallas, TX: Biblical Studies Press) 1998.
I would think that Paul is saying it is unprofitable to overly scrutinize the origins of meat (or of a ritual like the unity candle). If we are specifically informed that there are evil associations, then we should refrain, if only for the sake of a weaker brother. In all things, we should seek to act in a way that glorifies God.