i feel like its not clear enough so for those ppl:

ririko was sleeping with the yakuza guy and touma a lot before she met sentarou. despite that, she was in love with the yakuza guy and got heartbroken when he left for dubai. so when she realized that sentarou was also in love with someone and got his heart broken (even though he never confessed in the first place), she decided to go out with him because he was more innocent and cared about her. from the beginning, when she got pregnant, she wanted sentarou to be the father even if he wasnt because she knew he would actually care about megaru.
i think involving touma into the equation was ririko's way of giving sentarou his happy ending, because she eventually realized that that touma was the one that sentarou was in love with. so even though she could have just said it was sentarous child (because he would've agreed to raising megaru regardless of if touma was there or not), she said it was either sentarou or touma's child to get them to spend time together, almost like shes apologizing for being a bad girlfriend and leaving him with a kid who obviously isnt his.
the yakuza guy is megaru's bio dad and also, ririko was probably not actually lying when she said megaru's dad was in dubai or something, because she knew the yakuza guy was the dad.
+ sentarou was never actually in love with ririko. he always liked touma more, but chose to go out with ririko because he knew he wouldn't have a chance with touma. the reason why he cares about her so much is because hes a naturally kind and selfless person unlike touma, so really had more of an affection/friendshio towards ririko than romantic feelings.