I'm not 100% sure, but from all the talks it seems that it could've been the case of relationship becoming a long-distance one for a while (with Ian in Baluart and Gillian in their home city) with the intention of Gillian eventually coming to Ian, but she's apparently got entangled with the new guy during that time apart. It's unknown if this separation was because Ian didn't want to follow Gillian back or some other scenario. Either way it's possible that was just a typical case of things not surviving a separation and/or vulnerability that comes from the partner not being there for you when needed, than Gillian looking for "more adventures" per se.
(plus, just because Ian wasn't bad in bed doesn't mean the new guy couldn't have been even better.