Yaconelli bases his thesis in love, God said; love your neighbor as you love yourself. Yes indeed this is the most important message of the Bible. Before we go any further I would like to ask; What is love?
1 Corinthians 13:4-7
Love is patient, love is kind. It does not envy, it does not boast, it is not proud. 5 It does not dishonor others, it is not self-seeking, it is not easily angered, it keeps no record of wrongs. 6 Love does not delight in evil but rejoices with the truth. 7 It always protects, always trusts, always hopes, always perseveres. (1 Corinthians 13:4-7)
Yaconelli explains that there is nothing wrong in practicing love between two men, and two women, that if it's true love God is there and it is blessed. God created everything based on his image, God says "It is not good for the man to be alone" so he created women,
Genesis 2:18-24
21 So the Lord God caused the man to fall into a deep sleep; and while he was sleeping, he took one of the man’s ribs[b] and then closed up the place with flesh. 22 Then the Lord God made a woman from the rib[c] he had taken out of the man, and he brought her to the man.
23 The man said,
“This is now bone of my bones
and flesh of my flesh;
she shall be called ‘woman,’
for she was taken out of man.”
and flesh of my flesh;
she shall be called ‘woman,’
for she was taken out of man.”
24 That is why a man leaves his father and mother and is united to his wife, and they become one flesh.
Marriage is between a man and a women is for life, they promise love to each other until their last breaths. Marriage is the blessing of God. In the Bible indeed there is parts where it specifies that homosexuality is prohibited, just as is prohibited adultery.
1 Corinthians 6:9-10
9 Or do you not know that wrongdoers will not inherit the kingdom of God? Do not be deceived: Neither the sexually immoral nor idolaters nor adulterers nor men who have sex with men[a] 10 nor thieves nor the greedy nor drunkards nor slanderers nor swindlers will inherit the kingdom of God
Whether homosexuality is a sin or not, it is not my job to judge others. God is love, and he loves us so much that he gave us intelligence, he loves us so much that he allows us to make our own decisions. He loves us so much that even though we make decision against his rules, he still gives us the opportunity of the doubt. He has faith on us. 
