While I agree, MC was quite shocked and upset that his dad wasn't immediately supportive. I can give him a bit of a pass for now just because of how much of a shift that was from his dad's usual reaction and how much MC clearly idolized his dad. Hopefully MC's had time to process and can discuss it more calmly next time. Neil clearly realized quickly his error and probably would be open to at least listening.
But yeah, MC did a shit job explaining their relationship. Definitely failed to make it clear that 1) Josy and Maya were also in a relationship with each other and had been for a long time and 2) Josy and Maya were the ones to propose poly in the first place, not MC trying to convince two unrelated women to share. Understanding that really changes the way the relationship dynamic looks.
On a side note, I hope MC gets the option to encourage Maya and Josy to go public next episode. With the loan resolved, there's nothing explicitly holding them back except fear of the reactions from family and friends, and doing so can be really liberating and healthy for a relationship. It's hard to maintain feelings of love and affection when you're constantly having to hide and suppress them and the small gestures of expression like public kisses. Being free to do so would probably do a lot to help Josy and Maya.