...Except Sumika is definitely trying to make excuses because minus that brief period where they discovered they both have powerful libidos, they only have sex on the weekends for the most part. Sumika burns bread 80% of the time. We know from chapter 44 that they don't have sex literally every weekend night, which means they have sex less than 28% of the nights since they got married. Unless Sumika ONLY makes bread on mornings after they've had sex (which also discounts the long period they weren't sexually active), something doesn't add up; she's definitely burning the bread on other days when "I'm tired from the sex" isn't a possible explanation.
Conclusion: Sumika is just bad at toast, but knows she can act sexy to get away with it.