Is this cheating?

I recently discovered messages online between my man (of 3yrs) and another girl he has not met in person. In the messages he tells her she's beautiful and asks if she has a boyfriend. 
I'm caught completely off guard. I never would have expected this from him. Our relationship has been very good and I love him very much. But since discovering these messages I feel riddled with insecurity and am questioning our relationship. Help please :( 

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