There won't be any more talking scripture with you. It's all social reason. Youngling thought he could use "his authority" to explain "situation" to you. Now his social status is dashed as wave upon rocks.
As since you insist talking upon scripture and know better scripture than him, he is royally scr.....up. You see, legitimacy of church leadership is mostly because they know scripture better than most. And now you could replace them all (in their minds). After all, teaching sound doctrine is pastor's primary job and you could do it better than him.
And since congregation's best interest is having best teacher and if congregation finds out that you are better than pastor, well, then no jon security for current pastor.
Social assassination is something expected.
It wouldn't be first time I heard pastor being afraid of congregation member.
Why are at all trying to explain your in-laws correct scripture? Did they ask for it or show curiosity?
There is reason religion and politics are taboo topics. Give them 2 minute explanation why and STFU.
Did you even consider what effect your marriage has on their marriage? They do think should they copy you and what implications it will have on their marriage. How would mother-in-law who believes in hubby only for her react to husband of her daughter explaining to her husband that her hubby has absolute right to bring another women in her house without asking her anything? How about very not happy? And by the way, she can't get rid of you in legal way which means you could any time influence her hubby.
And best part is that social norms forbid them from having marriage fight in front of you. Therefore, best way to reduce conflict is to no longer speak about topic.
Don't be suprised if they go around your back to convince your wife you are wrong.
There is a lot of truth in what
@MemeFan says here. In his own abrasive fashion, he has torn aside the surface layer and exposed the true insecurities that probably are behind the reactions of both the pastor and the parents. To illustrate this:
Did you even consider what effect your marriage has on their marriage? They do think should they copy you and what implications it will have on their marriage.
I remember when we said we were leaving our family size up to God and ended up with a tonne of children, my grandfather got upset because he thought that meant I believed he was wrong to have only three. I had never said any such thing. But it is a good example of this truth - people take what you believe, apply it to their own lives, and then get offended at what that comparison means.
Do pay attention to what
@MemeFan is saying. You need to understand the root issues, and counter them. What people say is the problem is often not, and countering that is just countering a straw man. It doesn't matter how well you answer any of them scripturally, or how poorly, as scriptural disagreement is not the actual issue. It's just a disguise. They are cloaking their real worries in scripture to give them an appearance of legitimacy, but countering the scriptural argument does not address the real worries.
Your wife's parents, especially her mother (who will be the emotive influence on her father), needs reassurance that you will continue to treat her daughter well, and for her own sake needs reassurance that you do not believe her husband should take another wife against her objection. But that's hard because she doesn't even realise those are the issues herself, so you can't just address them directly. It is only time demonstrating that you're still a good husband which will persuade her heart.
Don't worry too much about scriptural argument. Just outlast this and demonstrate a great marriage. That is what will speak to their hearts, it just will take time. There is sadly no shortcut around that.