The friends are consensual, and they are simply inviting people to watch. It all sounds so welcoming, natural, wholesome, harmless.
Of course it seems that way. Any person worth their salt in the industry would be ashamed to admit it depended solely on being addictive, and missed out on the opportunities created by also saying it's basically harmless. A really good spin would be to say instead of being harmless, why, it's actually good for you!
The harm of inviting the (paying) world to watch as two people have consensual sex? Four, off top of my head:
1. The special bond from such intimacy is cheapened. Sex is wonderful - it doesn't just feel good, it involves another person that you have an opportunity to relate to in a unique way, and they to you. Properly used it involves dignity, selflessness, and deep relationship. I've heard others say similar things that cheapen it, like "it's just body parts touching each other." What a shame it's reduced to that.
2. The children that naturally come from this are treated as a negative aspect, instead of a blessing. What if a kid accidentally came from one of these videos, would you be able to tell him some day that your friends wanted to have him and it was totally worth it to stop the gravy train to do so once they found out? You, your friends, and I will not be around forever, but hopefully another generation will come after us - don't harm that generation by depriving them the foundation of knowing it is good that they are alive and the previous generation welcomed them.
3. What happens in the bedroom affects what happens in the house, and what happens in the house affects the culture. Stuff is connected - give me a culture that has honor in the bedrooms, because I know how well things will be going in the public sphere.
4. Lastly, there is a point that is crucial for Christians, but I think even non-Christians should be aware of it - it is harmful because instead of furthering the worship of God, it furthers a falsehood where the individual is worshiped instead. Let me speak of myself: just imagine what I would do if there were no God, and I were God for a day. It would not be a fun world for long, even for me. Can we really treat such thoughts as harmless, just inside our hearts and heads, having no bearing on each other and the world?