Deuteronomy 17:17 — God says David shouldn't collect many wives.
2 Samuel 5:13 — David violates the above order by collecting many wives/concubines.
1 Kings 15:5 — God deems David righteous; apparently he didn't do anything wrong.

Tell me what the Bible says about polygamy. It seems like it can't even make up it's own mind.

Otherwise, please note that you evaded my question about why you choose to ignore Lev. 20:13 and Rom. 1:32.

When you consider 2 Sam5:3 ,God gave command that any one that serves idols should be put to death.In the verse 5 God called it an act of wickedness.
In the verse 17 which you gave, God said"Neither shall he( the king) multiply wives to himself,that his HEART TURN NOT AWAY:neither shall he multiply to himself silver and gold........19 "that he may learn to fear the Lord God,to keep all the words of this law and these statutes to do them.
This indicates that this offence is not an offence purnishable by death unlike that of idolaters. God told us the reason why the king should not multiply wives,which was "that his heart turn not away from God and to fear the Lord God and keep his commandments.Those were the reasons God gave.

Now lets see whether David's heart, was, for once ,turned away from Lord God Almighty.
Psalm 17:3 reads" Thou hast proved mine heart;thou hast visited me in the night;thou hast tried me, & shall find nothing;I am purposed that my mouth shall not transgress.
Psalm 18:20-23"The Lord rewarded me according to my righteousness ;according to d cleanness of my hands hath he recompensed me.For I have kept d ways of d Lord and have not wickedly departed from God.For all his judgement were b4 me, and I kept myself from iniquity......
Psalm 25:12 " What man is he that feareth d Lord?him shall he teach in d way that he shall choose".

From the above passages of d bible and other numerous passages,we could see that the heart of David was never turned away from God.He never worshipped any idol.He feared d Lord God.He had faith in God.God loved him because despite the many wives he had,he did not forget God.

Although he had his shorcomings too,but he never justified himself,whenever he made mistakes he would quickly acknowledged his sin and ask for forgiveness unlike Saul ,the first king of Israel.

Remember what God said to Moses "I will have mercy on whom I will have mercy,and compassion on whom I will have compassion.Rom.9:15.

This is to say that if God did not count it as a sin against David because his heart was ever towards God,though he married many wives,does God not have the right to forgive him?
2Samuel 7:14-15 God told David concerning Solomon that "I will be his Father ,he shall be my son.If he commit iniquity,I will chasten him with the rod of men,& with d stripes of d children of men.But MY MERCY SHALL NOT DEPART AWAY FROM HIM,as I took it from Saul,whom I put away b4 thee.
So,if God has forgiven you of ur sins,you will be deemed righteous.
1kings15:5" because David did that which was right in d eyes of d Lord,he turned not aside from any thing that God commanded him all d days of his life,save only in d matter of Uriah the Hittite."
That is, mercy has authority to override judgement.

Marring more than one wife is not a sin,but it is a sin when the marriage creates problems for the children
,the wives and d society at large.Wicked men have exploited this to d detrement of their families.Most time children from a polygamous homes exhibits severe depression,anxiety,low self esteem,self destructive habits and violence.Polygamy gives children sense of lack of worth in a home where d father neglects the mother and d children in favour of new wife and her children.
That is why God is against polygamy.It is because of its after effect on d society and lives of d children.But if those prophets that married more than one wives could be faithful with their wives,it may be overlooked by God
Abraham had more than one wife too.It is not that it is bad or evil but it could be expoloited by wicked men.

But the act of homosexuality is a sin b4 God,God gave a direct judgement of death on them, as written in Lev20:13" If a man also lie with mankind,as he lieth with a woman,both of them have commited abomination:they shall surely be put to death.it is death also for anyman that lieth with a beast etc.

It is God that has the final say.He can use His love and mercy in favour of anyone as He wishes,and not body can ask Him why.