Heart = Engine. The heart is the body's engine. I it's not operating properly the body will not be able to perform. This includes but is not limited to climbing stairs and having sexual relations. Aside from that the same processes that cause plaque build up in the heart can also occur in the arteries supplying blood to the penis resulting in erectile dysfunction.
Doesn't. An unhealthy heart may lead to limitation in endurance and ability to perform work. These limitations may modify your sex life but do not necessarily translate into or lead to an unhealthy sex life.