I would do a little more research and possible seek out a physiotherapist that is a little more sympathetic. I don't think you should be considering giving it up at all yet. It's not like you are 60 years old.
Proper care and therapy/workout should keep your knees well if you do it properly.
This times 1000. When I tore 2 ligaments in my ankle this summer, the physiotherapist I went to was great. Although he specifically works with guys in the military and knows the various demands of the job, he was able to give me stretches and exercises to do that would effectively strengthen the joint and get me back to 100% as quickly as possible.
Do you have any clinics that specialize in sports medicine around you? There's a few in my city, and one is really good (The local OHL hockey team goes there) and I think that they could give you the most honest opinion, best advice and a solid recovery regimen to get you back on track to being able to ride.
EDIT: For another personal experience, my grandmother got a similar surgery about 8 years ago now I think. She regularly rides a stationary bike, and a bike out on the roads for exercise. Maybe look into getting a road bike or something to help strengthen stuff up?