That knowledge of monogamy is precisely what is making Ianto so confident. Now that he no longer has to worry about loosing Jack he feels bolder taking risks. And while he's not often much of an exhibitionist he absolutely loves the idea of everyone else who has kissed Jack this evening watching them right now. Its exciting and arousing and Jack is by no means the only one picturing him on his knees with his mouth wrapped around Ianto's cock in full view of the entire party right now.
He's rock hard by the time they pause for breath, and has no doubt Jack can feel his erection pressing against his thigh. That comment just makes him want him more. Never let it be said Jack doesn't have a way with words when it comes to sex. "I don't care who notices if we step out for a moment," He informs him hoarsely. Leaning closer he whispers boldly in his ear, "I want you to suck my brains out through my cock. Right now."