My wife and I were listening to a presentation from an insurance salesman she'd met somewhere - probably her work, and we were in our living room. She was interested in a threesome with him - we'd already discussed my little fetish which turned out to address one of her needs, so...... we agreed she'd let me know when she found the right guy. She asked the salesman to stay for a drink and gave me nonverbal signals he was the guy. As he fucked her, I could see from her response that there was a lot more than just sex going on in our bed. After he finished in her I quickly pulled her leg aside and moved to take her missionary, but I noticed I wasn't all the way hard. I was excited as all hell, but not all the way hard. With her help, I did manage to penetrate my wife and stiffened up as things went along. The finish was as it always is with immediate seconds with one's wife - intense, hot, and completely draining.
Afterwards, her new BF (soon to be lover) mentioned he'd seen I wasn't fully erect earlier, and told me that it sometimes happens to husbands whose wive's have regular BFs. As time went by and their relationship grew stronger my wife began having sex with him without me in the bedroom with them, and sometimes I'd be hard as hell knowing what was happening in our bed, but 50% of the time, I couldn't get hard enough to penetrate her. This went on for several years, but then (thankfully) my wife suggested I get some Cialis. I found it online without having to get an official prescription and things quickly improved - at the right times, my mojo was back!