Sure. This is absolutely a legitimate reason.
I actually registered to adopt when I had only been trying for about 2 years (adoption here takes about 6-7 years). As it happened, by the time 6 years were up I already had 3 children (a singelton & twins)... but it was nice to know that there was a child at the end of my path regardless of how my treatments turned out.
Good luck!
Rachel Inbar
www.FertilityStories.com