For years and years I pondered this question. How in the world could Christians be married to Christ? This passage particularly began to help me in understanding. (If only we would learn that all of our questions are answered in God's Word...)
1Know ye not, brethren, (for I speak to them that know the law,) how that the law hath dominion over a man as long as he liveth? 2 For the woman which hath an husband is bound by the law to her husband so long as he liveth; but if the husband be dead, she is loosed from the law of her husband. 3 So then if, while her husband liveth, she be married to another man, she shall be called an adulteress: but if her husband be dead, she is free from that law; so that she is no adulteress, though she be married to another man. 4 Wherefore, my brethren, ye also are become dead to the law by the body of Christ; that ye should be married to another, even to him who is raised from the dead, that we should bring forth fruit unto God. Romans 7:1-4
Paul says here that the law no longer has dominion over them (the Jews), just as a husband has dominion over his wife unless he dies, at which time she is loosed from the marriage. The Jews are no longer bound by the law, but rather they are now under the dominion of (married to) Jesus Christ, for the purpose of bringing forth fruit.
For more clarity on this you can read the previous chapter, Romans 6, where Paul talks about the kind of fruit they brought forth - sin - while bound to the law, and that they should now strive to produce a new and better kind of fruit, through the power of the Holy Spirit. To continue to produce sin is spiritual adultery. Examples that we can all relate to today are the apostate churches, which are committing spiritual harlotry. They claim to be Christian, but they act like the devil. All of us are susceptible to it, and this grieves the Holy Spirit.
Are you bringing forth good fruit?