It's only been a problem during the hot months when I like to walk around shirtless on the beach, poolside or around the house, especially early on in the summer before the tan takes hold. That's when they'll most likely notice, not during sex. But it all depends on how prominant these marks are against your skin tone and how much time you're allowing between sessions of sex with these different women. If you sleep with all of your plates in a single weekend and you're a fair-skinned guy, it's going to be a bigger problem than if you're darker and space sex a few days apart. This is really up to you to handle appropriately.
That said, in the throws of passion they won't keep count of how many times they dig their fingers into your back. If you can get them all to do it a little without drawing blood or leaving bad bruises, and one of them happens to catch a glimpse of your back one day, you can put it all on her. She'll be amazed with herself: "OMG! I did that?! Hee hee!!!"
Several reasons why I don't like it personally. One, it's an added expense I'd rather do without. Two, I don't want to carry two phones. Three, when you're spinning plates there is going to be some rotation going on: your main girl might bail, a side-plate becomes more of a main plate... whatever. And finally, pissed off girls love to explode your phone. Do you really want two of them?
Here's what I like about having only one phone:
If one girl calls while I'm with another and I don't answer it, it makes me look like I'm genuinely interested in my time with her; that I don't let my phone calls get in the way of our discussion or time together. The more it goes off and the more I don't answer it, the more guilty a woman I'm with will start to feel. Many of them even insist I take the call because they feel like they're getting in the way of my social life. In that case I step out, talk to one girl, finish up and then go back in and chill with the first.
Another thing I do is occasionally text back when I don't answer. I'll tell them I got their call but I'm busy, working, with a buddy, in a loud place, whatever, and that I'll call them back in a little while, later tonight or tomorrow. I do this fairly often, and many times my plates stop calling me out of the blue for BS, no-point, chit-chat just to see what I'm up to and will text me instead, asking if I'm available. I basically pick and choose my time to yack with them on the phone. I don't need two of them.
