To arrange male slave with female you need to go to Interaction and select your male and female slave in one sex mode (max 3 slaves + MC in one scene without Aphrodite Brew).
Yeah, that's another area where I feel this game falls a bit flat. People can not learn to have sex (and do most of the jobs in the game) without first having sex with the player. If you want to play an opposite-sex only or same-sex only mindset player (or if you want to only have sex with slimes, or with anyone except slimes), you still must ask everyone to have sex with you before they can agree to have sex with anyone else. You can force them into a group thing and sit idly by, but getting them to willingly submit is 1 on 1 only with player required as one of the 2. Sure, I guess one can get them to agree, and then just quit the sex scene without doing anything, and then suddenly they are available from then on. (On a separate note, since the upgrade that added in desire requests during sex, I feel like if my partner in a 1 on 1 scenario is going to keep yelling at me that they want multiple partners, then the game needs to give me a way to go out and add in an additional person part way through the sex scene)
Not to mention, since sex produces mana, and somehow it is a universal pool of mana for the household, so the owner doesn't even need to partake, why is there no sex version of the farm? Assign a group of people to having sex all day and building up my mana pool. Plus there could be a secondary group where someone is in charge of training slaves in the ways of sex. Assign people to the task of learning sex. (not like mana is particularly in short supply by mid-game, but still, seems a natural way for the game to go)
Heck, for that matter, the way mana is a joint-use pool, it should be made into a commodity. Instead of earning gold from brothels and so forth, one should get a percentage of the mana generated. Either do away with gold completely and have mana be the main currency, or allow buying and selling of mana.