Y'all, Ian’s “fuck, no” response was actually really sweet. This is going to be the moment where Ian really and truly starts thinking about marrying Mickey, and it makes perfect sense in the context of their life together.
The “fuck, no” is a knee-jerk reaction to a question he’s never had an opportunity to think about seriously before because of their circumstances. First, Mickey wasn’t out. Then, Ian’s illness got in the way. Mickey’s questionable choices about Sammi and subsequent prison sentence. Mickey escaping to Mexico. And finally, prison. All these experiences have conspired against them for years. Ian’s never believed marriage could be on the table with Mickey. It doesn’t matter that Ian mentioned it when they had the talk a couple of episodes ago. Yes, he has been thinking about his future with Mickey, and yes, marriage has at least been at the back of his mind. But Mickey immediately brushed that comment off, and anyway, Ian thinks he and Mickey are going to be separated for an extended period of time right now. I do believe they agreed to wait for one another (that’s exactly what Mickey was saying when he insisted “I’ll get out soon"—Mickey was telling Ian he would be coming home to him; the implication there was absolutely that they were going to wait for each other in Mickey and Ian speak). But that doesn’t mean that Ian’s not still uneasy about how everything is going to actually play out in the time they spend apart because shit happens and life gets in the way.
So, Ian’s just reacting here. He feels like his answer is supposed to be "no” because he and Mickey have never been on stable enough ground for marriage to be a possibility. But now, he’s going to have some time to rethink that—to realize that once Mickey’s out, they are both legitimately free. The previous barriers that stood in their way are long gone and it can just be the two of them, together, at home in the Southside, without all of the other crap hanging over their heads.
This is going to be the moment that helps Ian realize he and Mickey are finally in a place where they can have a stable relationship, a marriage, maybe kids someday—that he and Mickey can finally truly have a life together.
This moment was so important. It wasn’t bad writing. In fact, I think it was the exact opposite. This is what we call foreshadowing, and it absolutely makes sense in the context of Ian and Mickey’s story.