Is it a sin to see your future wives ?

my love is strong catholic and so am i but i got her to show me her s and i was grateful but 8 or so months later she told me that it was a sin and i dont want to make her sin but i dont mind no til marriage but that was a sin really? and on top of that she said it is a sin to get turned on but that is kinda hard when tthe love of my life is on top of me kissing me in alright but it ight that i get turned on by her easy she has my on and even if she doesnt mean to she smacks that on lol but i just need some help is it a sin