The Top Gun and L Word actress, who came out as a lesbian last year and wed long-term partner Melanie Leis months later, told The Times that she did not want to place the "burden" of her sexuality on her children.
McGillis explained: "I had a lot of shame issues attached to my sexuality. A lot. If Mel touched me in public, I would just freak out and go, 'Don't do that - stay 20 feet away from me'.
"It was also a burden on my children - it got to the point where [daughter] Sonora wouldn't allow Mel and me to come to her football matches together because she was so embarrassed.
"People say, 'Why didn't you come out earlier?' I just thought it would be incredibly selfish to put my kids through that misery because of something that I needed to do for me."
However, she said that her attempts to cover up her relationship were more destructive than helpful.
She added: "If I were more mature and healthy, I would have done it very differently, but I wasn't very well... I was completely self-absorbed. Ultimately, my children suffered a great deal because of it. But I can't take that back."