Preventing erection

You're probably wondering, "What the ...???"

Ok; here's the deal. My wife is (I think) somewhat curious about all this, or at the very least fantasizes (when she's horny) about doing a different guy. But, when we talk about it, she says she could never do anything like that, that I satisfy her already, etc. So, in the spirit of trying to cross one more item off the list, over the last several months, I've been trying to be less good in bed - eg, cumming quickly (which is not unusual), not penetrating her fully, not being fully hard.

It's that last item that's tough. I usually get her off with a big dildo, either before or after we go at it. She says that she really likes my thing too, but my logic is, if I can not be hard enough for penetration, she may start to wonder more about doing another guy.

So, does anyone have any "anti-viagra" methods?