You can't apply your cultural/community standard about sexual orientation and/or marriage on Japanese (and most oriental cultures actually) tradition. Of course Japan, China etc had been Westernized and Christianized to some extent, but I think it's safe to say JAV idols are more following the old school view that sex (all kinds) is not a big deal.
So it's perfectly possible a "legit" lez to suck and fuck cocks for a living, in fact may do straight sex more frequently (consider Yui's work load) than sex for her own wants (with man, woman or anything else). OTOH, it's just as likely she is "legit" straight (in her orientation) but currently in a same sex relationship.
Traditionally we (I am not Japanese, but oriental) don't think of sexual orientation the way Western/Christian think of it. For us, a person isn't gay or straight like the way s/he is tall or short, or left-handed or right-handed. A gay man, by default, would marry a woman and fuck her enough times to make at least one baby (prefer male baby) and perform all his duty as a man (and a husband) which doesn't mean he has to love his wife sexually. If he has the means, he can have a steady male lover or multiple. Marriage (only hetero is legit) and love/sex are two non-conflicting dimensions. A marriage's (almost) entire meaning is parenting (esp. having heir(s)), and extra-marital sex is comparable to drinking, it's only sinful if there is a negative consequences, like making a baby, or wasting away family fortune.
Most people have their sexual orientation defined by their circumstance rather than "self-discovery". So a lot of sex workers (prostitutes and by historical extension, AV idols) choose a lesbian relationship not because they love women in their "nature" but a same-sex partner is so much more convenient/compatible with their career, lifestyle and circumstance.
If you think that's mad, consider this well known social phenom: many (I'd say more than 30%) married couples in Japan don't have any physical contact with each other in a year, no kissing, no sex. They may talk to each other less than they talk to same sex friends/neighbor/co-workers. By default, a man's most important relationship is with his colleague, or his buddies. A woman's most important relationship is with her children. Love and romance is a just a life phase for teenagers until they get married and become parents.
So if Yui and Eririka declare themselves a couple, it means they (may) live together are together a lot. It doesn't mean they kiss or do lesbian sex, they may do it, but they don't have to. Physical intimacy is not a requirement for them to be a legit couple. Also it won't surprise me the circumstance will change and Yui's next relation is with a man.
Both are probably bisexual. Doesn't Eririka have some rich sugar daddy in Taiwan or something? Straight for pay seems like a pretty rare scenario to me anyway. Can't imagine a legit lesbian touching hundreds of dicks and working in the porn industry for as long as they have.