I'm quoting my memory of Anatomy & Physiology (the class, not the concept) so I might not be 100% correct, but its actually estrogen that causes growth of the long bones to occur, and a surge of estrogen, during late stage puberty in both males and females, that seals the growth plate (quite literally) and causes the bones to stop growing.
I'm not 100% on the role of estrogen in bone DENSITY, but just on its impact on the growth plate there has to be some.
You refer to the role exercise plays on bone health and that's true too. I think we started this tangent of the thread by saying "I wouldn't advise
just doing spiro for more than a year
because there's a almost 100% chance that you'll get osteoporosis out of that long term." And actually now that I've cut and pasted that, I've added the emphasis to just. Amazing what happens when you re-read something.

Back to bone denisties for a minute, you mention the role of muscle mass in building bone density. The simple answer would be adding even just light exercise should help counteract the osteoporosis effects when all else fails. Simply keeping the joints moving is good advice for anyone.
Back to the
just Spiro routine and I'll wrap this up. By definintion women have high levels of estrogen and low levels of testosterone (yeah there are always exceptions, but I'm trying to keep this simple), men have high levels of testosterone and low levels of estrogen. Sex hormones have more then just an impact on sexual behaviors and secondary characteristics. If you really want to know all of them, a web search will turn up a treasure trove. Nothing human has low levels of both. Not even pre-pubescent children.
On another note, sort of along the same lines, in researching my earlier post I came across some articles on the use of GnRH agonists (think blockers) to delay puberty in gender variant children. Current research is showing some level of success in its application, but there have been no long term studies, yet, to thoroughly validate its effectiveness. GnRH agonists got their start in treating early puberty in non-gender variant children, long term studies in that population cite some link to their use and infertility later in life.
The point is its pretty tricky to be messing with sex hormones for a number of reasons, and to block or inhibit one with out balancing it with its opposing hormone is incredibly risky on multiple levels (biologically, psychologically, etc).