This is one of my top 3 favorite episodes. I have always thought that the rays from the quasar, just provoked, if you will, the connection that already existed between Leela and Fry. They obviously already have some love for each other, and this episode and AOI show that it only takes a small change in circumstance for Leela to open up and accept Fry's advances.
Also, as far as sleeping with Fry goes in AOI, I have always thought that she sleeps with him because of the large amount of pent up emotions for him she has been hiding. When all these feelings come out so suddenly, lust in the end result. Wether that observation is the shipper in me coming out or not is unknown