Translation:They play together every Monday.
Here's my take: 'they are playing' is a present continuous, expressed in Gaelic as 'tha iad a' cluiche'. 'They play' is a repetitive, regular action, expressed in Gaelic, using a version of the future tense, as 'Bidh iad a' cluiche'. This is not so strange - the same is sometimes done in English: 'They will often go to the pub' as an example. To my ear it doesn't sound right to say 'They are going often to the pub' or 'They are playing every Monday'. Surely we say 'They play every Monday'. I do anyway!
It depends on the context. The ones you give sound fine to me, but in other contexts it sounds weird. The fishing one in particular. There's no exact translation of this construction really, because English doesn't use the future tense for this context. Considering the general preference for translating the present tense examples in the "I am playing" form rather than "I play" (for example in the early lessons the preferred translation of "Tha mi ag iarraidh" is always "I am wanting" with "I want" in the status of "another correct answer") it seems very odd to turn against the use of the same construction and mark it flat wrong in this context where I would argue that it's more natural for an English speaker.
They have different meanings though.
'I play' is a general phrase. No time is specified, just the action. It could mean something that is happening right now, or something that happens habitually. In English, this is the present simple tense.
'I am playing' is a phrase which specifies an action that is currently happening, right now. This is the present continuous tense.
Given these are two different tenses, they have two different meanings (even if they are often used interchangeably in English). You can't really get away with using them interchangeably in Gaelic.
- I play > Bidh mi a' cluiche
- I am playing > Tha mi a' cluiche
Hope that clears it up! :)