What constitutes as infidelity is subjective, and a lot of couples don't see eye to eye regarding it. XXX Church, an online ministry dedicated to helping people with porn and sex addictions, has weighed in on the matter.

Melissa Ruff writes on the ministry's website that other people should never be a part of anyone's intimate relationship. "Jesus even went a step further and said that having lust for someone when you look at them is infidelity," she says. "Adding people, having someone on the side, looking at other people having sex (on screen or off) will only damage your relationship. It destroys marriages more times than not."

When Ruff and her husband were struggling with pornography in their marriage, it led to problems and tore their sex life apart. For years, Ruff says they were too busy picking up the pieces of their porn infidelity. "Simply put, sex should only involve you and your spouse," she stresses.

Ruff adds that anything that can harm one's spouse physically or psychologically should be considered off-limits, too. Marital sex that thrives on being slapped or choked is demeaning, she says, and can make the other person uncomfortable. As for the psychological aspect, Ruff says neither spouse should ever feel like an object.

"Again – when porn was an issue in my marriage – I constantly felt objectified. Sex was only about him, and never about us," she says. "The way I thought about sex changed in a horrible way during those years. I equated sex to 'doing a chore.'"

Ruff says husbands or wives should never feel that way. Sex inside a marriage is a beautiful act because it makes both parties feel connected and loved.

So the bottom line is this: If either spouse is not hurting each other or adding another party, Ruff says they are free to do whatever they like.

"This is the part where you can explore each other without shame. Discuss what feels good and what doesn't. Talk about what you would like to try and what your spouse would like to try. Speak to your spouse openly and honestly. This intimate part of your relationship should be a safe place where both of you can be honest and vulnerable," she says.

Every couple is different, so it's never right to compare. Whenever there are issues, Ruff recommends that couples pray to God. "God is the creator of sex. He wants you to experience the full extent of enjoyment that sex brings," she says.