Retarded story made just for the sake to be annoying discarding all logic. Like every previous commenter said, many ways exist, condom, pill, finish outside, anal sex, non-penetration sex also works. The husband is a cuck and should be cuckolded.
Fortunately this moronic story genre is rom-com vanilla. Is this a hentai manga, the genre would definitely be NTR, the FMC would be fucked by her coach, or a member of the men volley team, most likely the captain, or one of her fans.