Dose she/he come back later from work?
Unfortunately, everything indicates that you are cheated.
Your partner’s behavior is very puzzling. Perhaps you do not give to her/him what they want? None of the betrayals does arise from itself. Firstly, you lack a common language, common dreams, and common needs. Secondly, each new day rather than shorten the distances from each other it makes no longer. Thirdly, you do not understand, and you may not want to understand the needs of your partner. In such a situation it is hardly surprising that he is looking for someone who will give him what you cannot give. It is not just only about sex, which is only a result of betrayal.
Perhaps you were not yet been betrayed, but your partner “has someone in mind.”
That someone provokes him, encourages, creates ambiguous situations, flirts, and thus approaching ever closer to your partner. Your husband or boyfriend, your wife, or girlfriend puts at stake the desire of betrayal on the one hand and the consequences that may arise because of his perfidy on the other. If you do not want to make this budding romance blossomed you need to start working! No screaming, no brawls, no excuses, but love can overcome what is being born somewhere outside of you and against you.
You have nothing to fear.
Your partner does not have an affair and doesn’t cheat on you. They do not even think about betrayal. He is happy with you to the extent that so far not even notice other people besides you. Demonstrate your love as often as possible, and do not treat sex as your duty towards your partner. Spend with him as much time, tolerate and respect his partner’s flaws as a human being. Your relationship has a chance to survive for years in harmony, and above all faithfulness.