First of all I will tell you (like other people already said) that aishiteru can be used for pretty much everything, unlike other forms of love.
愛 (ai): If you really love something you can use this term. You can use it on things, on activities, food, family, friendship, religion...
On the other hand other terms like 恋 (koi) can only be used towards someone you like because of your sexual orientation (so no family, friendship...)
So can aishiteru be used generally? Yes
Should you use it generally?:
Real life: No. People don't use it towards their friends or family because of how deep this love is. Basically A) people may misunderstand them B) They'd be too embarrassed to say it.
Don't say that to your japanese friends.
Anime: I have seen it a few times myself in anime/japanese novels without any sexual connotations. As I said before the term can be used generally so if the situation is clear (and unlike real life anime can be very clear) there won't be any misunderstandings and it can be used to highlight deep bonds.
For example, I remember watching a scene where 2 female best friends must part (one goes to another country). They are straight, have a boyfriend and yet they use the word ai. It was supposed to be a touching moment, showing how deep their friendship was. There was no possible misunderstanding.
Another example, in the VN Sharin no Kuni Sacchi uses that term towards her friend Mana. They're not blood related and they haven't known each other for that long either. Sacchi just loves Kenichi (in a romantic way), no one else. Saying that to Mana was used to highlight Sacchi's love for Mana (a deep bond), but there was nothing sexual oriented.
So the answer can be yes. Anime can portray emotions as black and white. There are even anime where the whole world is straight. If 2 girls love their boyfriends very much and their sexual orientation is that of the other sex (they are straight) then there is little room for misunderstandings. It would be used either to show how deep their bond is or as a gag if for example they're just classmates.