69ing is a totally unique experience. When both partners are in the right headspace for it, there are few things to me that are more erotic. There's nothing like the feeling of giving and receiving at the same time. I love the feeling of a woman's mouth around my cock and I also get insanely aroused eating a wet, swollen girl out. Both at once is overwhelming. Every thrust i make with my hips pushes her back against my mouth. It's a closed loop that drives me to cum harder than just about anything.
Not everyone's cup of tea obviously which I can respect. There are many easier, quicker ways to get either partner off - but in my opinion that's not the point of almost any partnered sexual activity.