This is from a manga, if that matters. It's written on the bottom of the last page of the story, so there is no further info on what happened.
I left that home and currently live alone. And "our thing" with Yuri-nee still continues. (I am/We are) seriously considering marriage.
So yeah, who is actually considering it? I'm not sure if a definite answer is possible, but perhaps one of the options seems more likely at least? I feel like it's "I am", but maybe in that case he should be considering プロポーズ since it's not clear how Yuri will answer? Or am I overthinking it?