hi, I don't think for one minute that there is a medical reason for no sex, as I have asked my nursing colleagues as I am now retired and not so up to date, if there would be any physical problems or concerns with a person having sex after an ileostomy or colostomy. Everyone did some checking and all have come back with the same conclusion its a big fat NO.
You are completely capable of enjoying the pleasure of intimacy in whatever way you find satisfying.
I think it might be an emotional response from your partner. It may not necessarily be because she is grossed out, she may be concerned that you could be hurt, or the bag could come away.
I would suggest that you sit down and have a very frank conversation and ask her what her concerns are and that you want her honesty and will not take personally anything that she has to say. This should reassure her that you are not just after sex but worried about her concerns or fears.
If she is grossed out or worried about the bag coming loose, then ask her if you could wrap yourself in ceran or glad wrap (the clear plastic on a roll that clings to itself). This will hold the bag secure and you could always wear a T shirt to cover it.
I am confident that if she is reassured that you won't come to any harm, and you wrap/cover up it won't be on display and she won't be so bothered by it.
Please try to understand her perspective and be patient but honest with each other.
I wish you well and good luck. Lorraine