Because even though they never loved another person when they meet again it seems the they wherent virg*ns

so they had s*xual reletionship with other people (its logical if you think the 13 years time skip)

author wrote that the FL only loved the ML in her life but in the 13 years she still was s*xual attracted to few other men during those years and dated them for a small time or once but never loved anyone and never had a successful relationship with someone and broke really quickly each time with those men because she couldnt open her heart to anyone else.

The ML too according a subordinate never loved another woman even though women chased him all the time because he couldnt forget the FL (ML had habit to say aloud the name of FL few times when he was drunk during the 13 years time skip)

Furthermore when they slept together for first time both seemed experience and didn't speak about first time etc.. so I assume that during the 13 years the slept with other people even though they never loved anyone else (purely s*xual relationships) at least that I understood author never said that they slept with other people directly so is somewhat open understanding from the reader.

and that irked me... I know that was the only logical conclusion with that unnecessary huge 13 year time skip but we read romance novel I would loved if the where stayed loyal even in body with each other and the time skip was much much smaller or no time skip at all.

plus

FL never stopped looking for ML she had a habit to looking in the for MC in the streets

and the ML even though he could find the FL sooner didnt want to meet her because of his underworld background