Women are always on the lookout for clues that indicate their partners are cheating. Any time you introduce an outside source of stimulation it sometimes raises their defenses and makes them suspicious. You need to let her know that your attention is always on her, that your focus is on her pleasure and not on seeking outside stimulation.
Compliment her, point out when other men notice her. Tell her how sexy she is and how other men think so. Tell her how sexy you find it when she sucks your cock, talk about her pleasure and how you love to make her feel good and how that is reward enough for you. Plant a seed that tells her you are focused on her and her pleasure.
When you point out how other men lust after her, make it fun, make it a situation that you can bring up to her in the throws of sex and tell her how it excites you to think of him having his way with her, giving her orgasms and feeling new (and better?) things.