According to the Bible, you cannot, nor should be understanding in this matter. Proverbs tells us that man cannot take fire to his chest and not be burned.... The same applies in this case
So you're saying a couple shouldn't be understanding of each other when one objects to doing something they find wrong or not something they want to do? I mean even Christian couples need understanding like that to function properly.
You stated two that are in love.... Which shouldn't even be gotten into.. Now for a couple already married, something will need to be done. I won't even go into what, because it differs with every case...
But for you as a single, non married person. Such a relasionship should not even be persued
Biblical references do tell you to stay away from relationships like this, but if you were to marry a non Christian, there is a reason for it, and its my opinion, that you married a non believer in hopes to one day win him over to Christ.
I am a living example of this. My wife is a Christian who married me when I was an unbeliever and I became a believer soon after. I believe that God worked through her in order for me to to find and accept Jesus Christ. If she was to never pursue a relationship with me I might have NEVER came to know the truth.
When we were looking for a pastor to marry us, we were turned down at first by my wife's childhood church because they would not marry a believer to unbeliever (yet two unbelievers were ok). We eventually found a pastor who was open to marrying us and believed that if we were to be married regardless, I should be given every opportunity to find Jesus. This display of love, to me, is what Christianity is all about and truly played a large role in my decision to believe.
I thank God every day that my wife was patient and had the hope that one day I would know the truth.