You can't control how your partners' other relationships develop, but you can control how you allow them to intersect with and affect your life. You are allowed to set boundaries on your personal space and time. You don't have to make the first time you hang out with your husband and his girlfriend be a public appearance at a crowded party. You don't have to be okay with hearing them have sex, now or ever. Take care of yourself so you can take care of those around you.
