I don't see this as a male or female thing at all. It's a boundaries issue between two individual human beings, nothing more, nothing less.
Boundaries come up every so often in every friendship and relationship, either by what is said, something that is done, or even just a simple remark or comment. I think each of us in our own minds draws a line over what is appropriate in a friendship or relationship based on what we know of ourselves and the other person. Everything is cool when it's all within the lines but when it crosses the line that's when it becomes a problem.
It's how you respond when something crosses the line which determines what happens next. You can either let it go and let the friendship or relationship develop further, you can discuss it and share the boundary so it becomes a boundary for the other person as well, or you can end the friendship or relationship.
I'm of the opinion that most people make being trans out to be a far bigger issue than it is in reality. That's what prevents us many times from getting that job, or someone entering into a friendship or relationship with us. They see the 'trans' label, they see it as a major issue, something which they'd rather not deal with, and they're unwilling to see past the issue.
But even when the cisgendered are in a friendship and relationship with us it remains a major issue for them, particularly when it comes to boundaries. Looking at it logically there's just two genders - male and female - so it should be fairly simple, right?
But no it isn't, because to them being trans is a major issue, something the cisgendered have real problems trying to relate to and empathize with, and they often feel that they cannot assume or anticipate where the boundaries are in a friendship or relationship.
But the same is true for us being trans. I feel it's got to be said that some of us make being trans a bigger issue than it is in reality.
But to me being trans doesn't change the fact that one of the biggest factors which determines how other people are going to treat you and relate to you is how you choose to treat them and relate to them. On this point there's no difference between anyone, irrespective of whether they are trans or cis.
If you make being trans a major issue, then so too will everybody you come across.
I tell people that my being trans isn't really all that major or important to me, it's just a label - nothing more - to indicate that I'm female but wasn't born female. I have no desire to be a natural born woman, or to be equal to a natural born woman, because I'm comfortable with who I am and would rather they treated me as they would any individual woman.
I do this as a way of trying to reduce the great big trans issue down to what I feel is it's normal size, just as trivial as having diabetes, or green eyes, or blonde hair. This I've found allows the other person to feel that they can relate to me pretty much the same as any woman.
When it does become an issue I tend to either make a joke of it or dismiss it. I once worked as a volunteer in a night shelter for the homeless and once very successfully and very effectively stopped a fight between a British guy and a Polish guy. I've lived in Poland for many years and speak fluent Polish. These two guys were swinging punches at each other and me as a just under six foot 300lb transsexual woman just completely stunned them. Then someone quipped 'My God, a woman with balls.' and we all laughed. The nickname stuck and I became known as 'the woman with balls'.
I think being honest your friend is probably feeling unsure of the boundaries in your friendship because he sees the trans issue as something major. This is probably why he caught himself or feels that he will offend or upset you.
Reading the thread has made me aware of just how difficult this issue can be if you're FtM in a friendship with a cismale where crossing a boundary inappropriately can quickly lead to a conflict.
I don't think that your friend is doing this intentionally, but is probably unsure of where the boundaries lie in this part of your friendship.
I think much depends on where you decide you stand on this issue and how you want the friendship to develop. You can let it go, and cross the boundary, in which case your friend will assume that such jokes are acceptable. You can discuss it, and share the boundary, so that you friend knows that it's off limits or that making such comments hurts you and upsets you. Or you could simply walk away.
I don't see it as a gender issue because neither of you are generic, but two individual human beings.