The most common reason for this belief is, naturally, Haruka and Michiru's determination to murder Hotaru in the manga. They refuse to accept Sailor Moon's offer of help and they refuse to allow the inner soldiers to interfere in the completion of their mission. They don't feel that they have to explain their reasons and come off as being very cold and unforgiving. In the manga, they even go as far as attacking the younger girls in an effort to make sure they stay away.
This is all a ruse. While Haruka and Michiru do intend to kill Hotaru, it's not something they take lightly. They pity Hotaru greatly but they believe that sacrificing one life is worth it to save the lives of billions. They have seen first-hand the destruction that Sailor Saturn is capable of and will stop at nothing to save the world, even if it means sacrificing their own lives. Their cold attitudes toward the younger girls is not because they feel that they are inferior soldiers, but because they don't want them to have to deal with the burden of taking a life. Usagi is a pure hearted girl who can't stand to see anyone suffer. Haruka and Michiru have such a strong desire to protect Usagi that they will not allow her or her friends to get involved in something that would break their hearts or get them killed in the process. So they do what they can to try to scare the girls off.
In the anime, Haruka and Michiru come off as being a lot colder because they constantly look at pure heart crystals with the intent of letting the owner die if they find out the crystal contains a talisman. They often outright shun the inner soldiers and often refuse to help, even fighting Sailor Moon and Tuxedo Mask to get their hands on a pure heart. Of course, their motives are the same as they are in the manga; they accept that they have to sacrifice 3 people in order to save the lives of everyone on the planet. They are cold and aloof because they don't want anyone else to have to share the burden of murdering three pure-hearted individuals.
Neither woman is selfish in any respect, except maybe when it comes to each other. Again, this stems mostly from the anime. Haruka is seen as being selfish because she refuses to accept her destiny, leaving Michiru to fight alone and Michiru is seen as being selfish because she wants Haruka to join her. There are also several instances in the fourth and fifth seasons where they would choose each other over saving the world.
Michiru does admit to wanting Haruka to join her but feels guilty about it and actually stops Haruka from picking up her Lip Rod, warning her that her life will never be the same once she picks it up. Meanwhile, Haruka only accepts her destiny as a sailor soldier when Michiru is badly injured from saving her. She makes up her mind to not let Michiru fight alone after she learns just how much Michiru cares about her.
As for choosing each other over saving the world, it's important to note that these instances take place after Sailor Saturn is reborn. Prior to this, Haruka and Michiru were trying to avoid having a real relationship with each other. We see them about to hold hands and pull away a few times, because they think it would hurt too much if they openly accepted each other's love. It's harder to sacrifice your loved one than it is your partner. I believe that after their initial mission is complete, they are able to freely love each other and won't let much stand in the way of that. Of course, I also don't really believe that they would really sacrifice the world for each other; most of these instances are either flirtatious boasting or situations where they knew they would not lose or where there would not be any serious consequences. Michiru has some pretty strong insight, you know! ♥