Its been interesting reading peoples thoughts on Aaron's insecurities and in this particular case relating to Robert, if I repeat what others have said sorry.
Aaron has not really been that confident in himself although much improved since he came out as he matured and went in to his first relationship with Jackson.
With Robert his relationship was as the lover knowing that Robert was engaged, getting and eventually got married and kept telling Aaron he loved Chrissie. Aaron then discovers that Robert also had an affair with Chrissie's sister so he is a serial cheater and quite brazen about it as well, once with a sister and the second time with a man its like the ultimate slap in the face. So Aaron knows Robert has a track record of cheating, he knows Robert is Bisexual and he knows of two occasions where Robert cheated on Chrissie (him being one of them). Therefore, Aaron WILL have niggling doubts, its human nature, Robert does have to prove he will be faith but Aaron also has to allow Robert to be Robert and if they are to be a successful married couple he needs to trust Robert, Robert on the other hand has to respect Aaron and if he is getting in to something "Robertesque" then he need to forewarn Aaron to avoid conflict.
Thats my thoughts anyway