Warning: slightly graphic content.
Yes it is, for the same reason that watching a fictional sex scene in a movie with the purpose of arousing yourself would be, even if the act was only ever performed with your husband.
Either way, it is introducing a third (indeed a fourth, fith, sixth and whatever, depending on what you are writing about) ‘person’ into your relationship when there should be only two of you.
The purpose of sexuality is to ensure unity - real unity, mental as well as physical, between husband and wife. In what sense is your unity real if you are imagining people other than your husband in order to ‘get in the mood’?
And is that not what you are doing - regardless of whether the imaginings and fantasies are real or fictional? Are you not just opening yourself up for serious dissatisfaction with your sex life down the track, since reality can never possibly measure up to fantasy? In fact are you not doing exactly the same thing that someone who masturbates is - only substituting your husband’s body for the right hand or appliance or whatever you might use if you masturbate?