You are very brave to speak the truth here.
We are not saints, so it is very common to have sexual fantasies… you have it,
I have it, even priests and sisters have it.
And the sexual fantasies will not care whether that object is married or not.
I remember Sister Faustina Kowalska wrote in her diary 《Devine Mercy in my soul》, that it is until at some moment in her life , she had a vision that Jesus put a golden robe
around her waist, that she is no more tempted by sexual sins from then on – which means, as a great Saint, she still faces the same temptations as us.
So please do not blame yourself too much . We all meet the same problems.
And perhaps the priest who is listening to your confession is facing the same temptations too…
I believe that it is better to be open and honest, and tell about the real temptations we meet, instead of hiding it ----- in the history , some priests and sisters hide it , which caused scandals in catholic history.