I read through all the responses and only saw fietsbob slightly refer to this. So I'll say it...I think it's the beginning of what will eventually become a hip replacement for you. I had the same exact problem with mounting. I didn't have any pain (yet) but just a loss of flexibility and had trouble swinging my leg (either leg) over. While looking at x-rays of my lower back, my chiropractor noticed the degeneration in my hip and said "you'll need to have that replaced soon". It was about a year before I began to feel pain, but had steadily lost more flexibility and had to start mounting/dismounting like fietsbob...laying the bike on the ground and then picking up between my legs. I got the hip replaced in March of this year and can again mount my bike like a younger man.

Dan